I, too, was annoyed by this, and here is how I fixed it.
Place this line in the control.keypress event:
NODEFAULT
While this control has the focus, the keyboard will not produce any effect whatsoever. Not quite what you wanted? Add this line above the NODEFAULT line:
if nKeyCode=9
return
endif
Now, if you press TAB, it will allow the key, any other key it will not. You can replace the number 9 with any other key code and even string them together with a bunch of 'OR's. For the list of key codes, look in the MSDN library for the INKEY() command.