Sorry about that.  The theory was sound, but it turns out (unbeknownst to  
me) that the TextBox class does not actually do its real work in the  
OnMouse... methods.  After thinking about it a moment, I realize that this  
may be because the TextBox class doesn't really implement most of the  
behavior, but instead just uses the built-in Windows text edit control.

With that in mind, I looked to see if you could accomplish the same thing  
at a lower level, and lo and behold you can.  Here is some code that does  
basically what I was talking about (it goes in the control class derived  
from TextBox, and you need to use the derived class as the control in your  
form):

        const int WM_MOUSEMOVE = 0x0200;
        const int WM_LBUTTONDOWN = 0x0201;
        const int WM_LBUTTONUP = 0x0202;

        protected override void WndProc(ref Message m)
        {
            bool fHandled = false;

            switch (m.Msg)
            {
                case WM_LBUTTONDOWN:
                    // Ignore the left mouse button down event
                    fHandled = true;
                    break;
                case WM_MOUSEMOVE:
                    // Ignore any mouse move events
                    fHandled = true;
                    break;
                case WM_LBUTTONUP:
                    // Before we process the left mouse button up event,
                    // send a left mouse button down event with the same
                    // mouse location as the mouse up event, to simulate
                    // an actual click at the location.
                    Message mT = m;           // copy the current message
                    mT.Msg = WM_LBUTTONDOWN;  // change the message ID
                    base.WndProc(ref mT);     // send the simulated message
                    break;
            }

            if (!fHandled)
            {
                base.WndProc(ref m);
            }
        }

Because this version of the implementation operates on the underlying  
Windows messages themselves, it means that even when the text selection is  
being handled by a non-.NET piece of code, the mouse messages are still  
intercepted before that code handles them.

Note that the above disables selection, but allows the user to reposition  
the caret.  If you want no mouse interaction at all, just set "fHandled"  
in the WM_LBUTTONUP case as well, and take out the code that's in that  
case now.

Also note that the above is really just proof-of-concept.  I have not  
verified that hiding the mouse messages like this does not also result in  
some other funny behavior, that may or may not be desired.  You should  
think about the exact behavior and do thorough testing before using  
anything like this in real-world code (especially since I did neither :) ).

Hope that helps.

Pete