Hi,

Override the WndProc of the control, and handle the WM_PAINT message. When
handling the message you want to check if the data in the clipboard is valid
and allow normal processing, otherwise don't let the base control get the
message.

For example:

Public Class CustomTextBox
   Inherits System.Windows.Forms.TextBox

   Private Const WM_PASTE = &H302

   Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)

       ' check for WM_PASTE message
       If m.Msg = WM_PASTE Then

           Dim i As IDataObject
           Dim s As String = ""

           ' get data currently in the clipboard
           i = Clipboard.GetDataObject()

           ' check for text
           If i.GetDataPresent(DataFormats.Text) Then
               s = CType(i.GetData(DataFormats.Text), String)
           End If

           ' exit if invalid data in the clipboard
           If IsInvalid(s) Then Return

       End If

       ' default processing
       MyBase.WndProc(m)

   End Sub

   Private Function IsInvalid(ByVal s As String) As Boolean

       ' put your logic here
       Return (s = "can't paste this")

   End Function

End Class

Regards,

   Gabriele