Hi mystique
Try this...
U must implement the IDocHostUIHandler on your form.

Declare the following Interfaces And structurs.
-------------------------------------------------------------
using System;
using System.Diagnostics;
using System.Windows.Forms;
using System.Runtime.InteropServices;

// IDocHostUIHandler interface

public enum DOCHOSTUIFLAG
{
    DOCHOSTUIFLAG_DIALOG = 0x1,
    DOCHOSTUIFLAG_DISABLE_HELP_MENU = 0x2,
    DOCHOSTUIFLAG_NO3DBORDER = 0x4,
    DOCHOSTUIFLAG_SCROLL_NO = 0x8,
    DOCHOSTUIFLAG_DISABLE_SCRIPT_INACTIVE = 0x10,
    DOCHOSTUIFLAG_OPENNEWWIN = 0x20,
    DOCHOSTUIFLAG_DISABLE_OFFSCREEN = 0x40,
    DOCHOSTUIFLAG_FLAT_SCROLLBAR = 0x80,
    DOCHOSTUIFLAG_DIV_BLOCKDEFAULT = 0x100,
    DOCHOSTUIFLAG_ACTIVATE_CLIENTHIT_ONLY = 0x200,
    DOCHOSTUIFLAG_OVERRIDEBEHAVIORFACTORY = 0x400,
    DOCHOSTUIFLAG_CODEPAGELINKEDFONTS = 0x800,
    DOCHOSTUIFLAG_URL_ENCODING_DISABLE_UTF8 = 0x1000,
    DOCHOSTUIFLAG_URL_ENCODING_ENABLE_UTF8 = 0x2000,
    DOCHOSTUIFLAG_ENABLE_FORMS_AUTOCOMPLETE = 0x4000,
    DOCHOSTUIFLAG_ENABLE_INPLACE_NAVIGATION = 0x10000,
    DOCHOSTUIFLAG_IME_ENABLE_RECONVERSION = 0x20000,
    DOCHOSTUIFLAG_THEME = 0x40000,
    DOCHOSTUIFLAG_NOTHEME = 0x80000,
    DOCHOSTUIFLAG_NOPICS = 0x100000,
    DOCHOSTUIFLAG_NO3DOUTERBORDER = 0x200000
}

public enum DOCHOSTUIDBLCLK
{
    DOCHOSTUIDBLCLK_DEFAULT = 0,
    DOCHOSTUIDBLCLK_SHOWPROPERTIES = 1,
    DOCHOSTUIDBLCLK_SHOWCODE = 2
}

// MSG structure
[StructLayout(LayoutKind.Sequential)]public struct MSG
{
    public IntPtr hwnd;
    public int message;
    public int wParam;
    public int lParam;
    public int time;
    public tagPOINT pt;
}
[StructLayout(LayoutKind.Sequential, Pack=4)]
public struct tagRECT
{
    public int left;
    public int top;
    public int right;
    public int bottom;
}

[StructLayout(LayoutKind.Sequential, Pack=4)]
public struct tagPOINT
{
    public int x;
    public int y;
}

// DOCHOSTUIINFO structure
[StructLayout(LayoutKind.Sequential)]public struct DOCHOSTUIINFO
{
    public int cbSize;
    public DOCHOSTUIFLAG dwFlags;
    public DOCHOSTUIDBLCLK dwDoubleClick;
    public string pchHostCss;
    public string pchHostNS;
}

// OLEMENUGROUPWIDTHS structure
[StructLayout(LayoutKind.Sequential)]public struct OLEMENUGROUPWIDTHS
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=6)]public int[] width;
}

[StructLayout(LayoutKind.Sequential)]public struct OLECMDTEXT
{
    public UInt32 cmdtextf;
    public UInt32 cwActual;
    public UInt32 cwBuf;
    public char rgwz;
}

[StructLayout(LayoutKind.Sequential)]public struct OLECMD
{
    public long cmdID;
    public UInt64 cmdf;
}

// Interop definition for IOleCommandTarget.
[ComImport(), Guid("b722bccb-4e68-101b-a2bc-00aa00404770"),
InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]public interface
IOleCommandTarget
{
    void QueryStatus (ref Guid pguidCmdGroup, UInt32 cCmds, [
MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1) ]OLECMD prgCmds,
ref OLECMDTEXT pCmdText);

    void Exec (ref Guid pguidCmdGroup, long nCmdId, long nCmdExecOpt, ref
object pvaIn, ref object pvaOut);
}

// ICustomDoc interface
[ComVisible(true), ComImport(),
Guid("3050f3f0-98b5-11cf-bb82-00aa00bdce0b"),
InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]public
interface ICustomDoc
{
    void SetUIHandler (IDocHostUIHandler pUIHandler);
}

// IOleWindow interface
[ComVisible(true), ComImport(),
Guid("00000114-0000-0000-C000-000000000046"),
InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]public
interface IOleWindow
{
    void GetWindow (IntPtr phwnd);
    void ContextSensitiveHelp (bool fEnterMode);
}

// IOleInPlaceActiveObject interface
[ComVisible(true), ComImport(),
Guid("00000117-0000-0000-C000-000000000046"),
InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]public
interface IOleInPlaceActiveObject : IOleWindow
{

    // IOleWindow interface
    new  void GetWindow (IntPtr phwnd);
    new  void ContextSensitiveHelp (bool fEnterMode);
    void TranslateAccelerator (ref MSG lpmsg);
    void OnFrameWindowActivate (bool fActivate);
    void OnDocWindowActivate (bool fActivate);
    void ResizeBorder (ref tagRECT prcBorder, IOleInPlaceUIWindow
pUIWindow, bool fFrameWindow);
    void EnableModeless (bool fEnable);
}

// IOleInPlaceUIWindow interface
[ComVisible(true), ComImport(),
Guid("00000115-0000-0000-C000-000000000046"),
InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]public
interface IOleInPlaceUIWindow : IOleWindow
{

    // IOleWindow interface
    new  void GetWindow (IntPtr phwnd);
    new  void ContextSensitiveHelp (bool fEnterMode);
    void GetBorder (ref tagRECT lprectBorder);
    void RequestBorderSpace (ref tagRECT pborderwidths);
    void SetBorderSpace (ref tagRECT pborderwidths);
    void SetActiveObject (IOleInPlaceActiveObject pActiveObject, string
pszObjName);
}

// IDropTarget interface
[ComVisible(true), ComImport(),
Guid("00000122-0000-0000-C000-000000000046"),
InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]public
interface IDropTarget
{
    void DragEnter (IDataObject pDataObj, int grfKeyState, tagPOINT pt,
ref int pdwEffect);
    void DragOver (int grfKeyState, tagPOINT pt, ref int pdwEffect);
    void DragLeave ();
    void Drop (IDataObject pDataObj, int grfKeyState, tagPOINT pt, ref int
pdwEffect);
}

// IOleInPlaceFrame interface
[ComVisible(true), ComImport(),
Guid("00000116-0000-0000-C000-000000000046"),
InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]public
interface IOleInPlaceFrame : IOleInPlaceUIWindow
{

    // IOleWindow interface
    new  void GetWindow (IntPtr phwnd);
    new  void ContextSensitiveHelp (bool fEnterMode);

    // IOleInPlaceUIWindow interface
    new void GetBorder (ref tagRECT lprectBorder);
    new void RequestBorderSpace (ref tagRECT pborderwidths);
    new void SetBorderSpace (ref tagRECT pborderwidths);
    new  void SetActiveObject (IOleInPlaceActiveObject pActiveObject,
string pszObjName);

    // TODO: hmenuShared should be a reference to a remotable handle
    void InsertMenus (IntPtr hmenuShared, ref OLEMENUGROUPWIDTHS
lpMenuWidths);
    void SetMenu (IntPtr hmenuShared, IntPtr holemenu, IntPtr
hwndActiveObject);
    void RemoveMenus (IntPtr hmenuShared);
    void SetStatusText (string pszStatusText);
    void EnableModeless (bool fEnable);
    void TranslateAccelerator (ref MSG lpmsg, UInt16 wID);
}

// IDocHostUIHandler interface
[ComVisible(true), ComImport(),
Guid("bd3f23c0-d43e-11cf-893b-00aa00bdce1a"),
InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]public
interface IDocHostUIHandler
{

    [PreserveSig()] int ShowContextMenu(int dwID, ref tagPOINT ppt, [
MarshalAs(UnmanagedType.IUnknown) ]object pcmdtReserved, [
MarshalAs(UnmanagedType.IDispatch) ]object pdispReserved);

    void GetHostInfo (ref DOCHOSTUIINFO pInfo);

    [PreserveSig()] int ShowUI(int dwID, IOleInPlaceActiveObject
pActiveObject, IOleCommandTarget pCommandTarget, IOleInPlaceFrame
pFrame, IOleInPlaceUIWindow pDoc);

    void HideUI ();
    void UpdateUI ();
    void EnableModeless (bool fEnable);
    void OnDocWindowActivate (bool fActivate);
    void OnFrameWindowActivate (bool fActivate);

    void ResizeBorder (ref tagRECT prcBorder, IOleInPlaceUIWindow
pUIWindow, bool fFrameWindow);

    [PreserveSig()] int TranslateAccelerator(ref MSG lpMsg, ref Guid
pguidCmdGroup, int nCmdID);

    void GetOptionKeyPath (ref string pchKey, int dw);

    void GetDropTarget (IDropTarget pDropTarget, ref IDropTarget
ppDropTarget);

    void GetExternal ([ MarshalAs(UnmanagedType.IDispatch) ]ref object
ppDispatch);

    [PreserveSig()] int TranslateUrl(int dwTranslate, ref UInt16 pchURLIn,
IntPtr ppchURLOut);

    [PreserveSig()] int FilterDataObject(IDataObject pDO, ref IDataObject
ppDORet);
}

-----------------------------------------------------------------------------

On the form (which having the webbrowser control)
make it implement from IDocHostUIHandler

And Implement the following Methods.
-----------------------------------------------------------------------------
public void EnableModeless (bool fEnable)
        {
            //Do nothing
        }
        public int FilterDataObject(System.Windows.Forms.IDataObject pDO, ref
System.Windows.Forms.IDataObject ppDORet)
        {
            //Do nothing
            return S_FALSE;
        }
        public void GetDropTarget (IDropTarget pDropTarget, ref IDropTarget
ppDropTarget)
        {
            //Do nothing
        }

        public void GetExternal (ref object ppDispatch)
        {
            //Do nothing
        }

        public void GetHostInfo (ref DOCHOSTUIINFO pInfo)
        {
            pInfo.cbSize = pInfo.ToString().Length;
            pInfo.dwFlags = DOCHOSTUIFLAG.DOCHOSTUIFLAG_NO3DOUTERBORDER |
DOCHOSTUIFLAG.DOCHOSTUIFLAG_NO3DBORDER |
DOCHOSTUIFLAG.DOCHOSTUIFLAG_FLAT_SCROLLBAR;
            pInfo.dwDoubleClick = DOCHOSTUIDBLCLK.DOCHOSTUIDBLCLK_DEFAULT;
        }
        public void GetOptionKeyPath (ref string pchKey, int dw)
        {
            //Do nothing
        }

        public void HideUI ()
        {
            //Do nothing
        }

        public void OnDocWindowActivate (bool fActivate)
        {
            //Do nothing
        }
        public void OnFrameWindowActivate (bool fActivate)
        {
            //Do nothing
        }

        public void ResizeBorder (ref tagRECT prcBorder, IOleInPlaceUIWindow
pUIWindow, bool fFrameWindow)
        {

        }
        public int ShowUI(int dwID, IOleInPlaceActiveObject pActiveObject,
IOleCommandTarget pCommandTarget, IOleInPlaceFrame pFrame,
IOleInPlaceUIWindow pDoc)
        {
            //Do nothing
            return S_FALSE;
        }

        public int ShowContextMenu(int dwID, ref tagPOINT ppt, object
pcmdtReserved, object pdispReserved)
        {
            //Do nothing
            return S_OK;
        }
        public int TranslateAccelerator(ref MSG lpMsg, ref System.Guid
pguidCmdGroup, int nCmdID)
        {
            return S_OK;
        }
        public int TranslateUrl(int dwTranslate, ref System.UInt16 pchURLIn,
System.IntPtr ppchURLOut)
        {
            //Do nothing
            return S_FALSE;
        }
        public void UpdateUI ()
        {
            //Do nothing
        }
------------------------------------------------------------------------------
private void SetUIHandler()
{
    //                            browser control Name
       ICustomDoc icd = (ICustomDoc) WebBrowser.Document;
    icd.SetUIHandler(this);
}

call SetUIHandler function in Form's Load Event
------------------------------------------------------------------------------

The Key behind this is overriding the ShowContextMenu function and
return S_OK.
S_OK means the user display his own menu.
http://msdn.microsoft.com/workshop/browser/hosting/reference/ifaces/idochostuiha
ndler/showcontextmenu.asp

Regards
Prakash