I don't want the user of my applications to enter text into a GtkCombo. Any idea?

A GtkCombo has an associated entry which can be accessed using the following expression:

      GTK_COMBO(combo_widget)->entry

If you don't want the user to be able to modify the content of this entry, you can use the gtk_entry_set_editable() function:

      void gtk_entry_set_editable(GtkEntry *entry, 
                                  gboolean editable);

Set the editable parameter to FALSE to disable typing into the entry.