As a side note, there's also the standard windows Close button at the top right of the Access window itself, which (AFIAK) can't be altered with the Ribbon.
There is some Windows API code that can be used to disable the button, but I tend to think it's a bit of a hack myself (changing the core UI things that people expect across every application doesn't usually do anyone any favors).
I won't bother to dig up the API code to do it as you don't seem to need it, but future readers may be interested to know and can probably find it with a little google-fu easily enough (or post back and someone will dig it up, probably in Stephen Lebans' or Dev Ashish's old codebanks).
Also - didn't the MS Access team add an Application Close event a year or two back? I seem to recall something along the lines, but not sure details offhand (ideally, this would let you do a little bit of cleanup prior to app close, because otherwise the only way was with a hidden form on startup, catching that close event, at which point Access is already half-shut down anyway - it's pretty messy). On a somewhat related note, I wrote an article about the weirdness of form events: Close, and Dirty, specifically:
http://www.utteraccess.com/wiki/Form_Event_Quirks
Cheers