Open up config\mamewah.cfg in notepad, that where all the keybinds are assigned.
Just remove the MENU_SHOW keybind, or maybe put one on for MENU_LOCK that isnt mapped to your board? I havent tried that option, not sure what it does.
The NEXT_PLATFORM and PREVIOUS_PLATFORM fields are there to scroll through, Im not sure what the SELECT_PLATFORM key does, so that may do exactly what you were asking for at the start, check it out!