Can I Disable or Alter GtkEntry and GtkTextView Clipboard Menus?

["Vallone, Anthony" <anthony vallone lmco com>]

I am developing applications that run in a very user controlled
environment (think big brother).  When a GtkEntry or GtkTextView widget
is clicked with button 3, a clipboard menu opens with basic clipboard
options, an "Input Methods" sub menu, and an "Insert Unicode ..." sub
menu.  How can I disable or alter that menu?  
 
I tried connecting to the "populate-popup" signal using
g_signal_connect() and g_signal_connect_after(), but what can I actually
do in the callback?  From within the signal callback, none of these seem
to have any effect:  gtk_menu_popdown(), gtk_menu_shell_deactivate(),
gtk_menu_shell_append().  The only thing I tried that actually prevents
the menu from opening is gtk_widget_destroy(menu), but that can't be the
only solution.
 
I also tried connecting to the "popup-menu" from the GtkWidget level,
but that signal never seems to propagate.

Thanks,
Anthony