Is it possible to disable Expose programmatically?
Read this article for some tips how to programmatically disable some osx features. It's a workaround (since it just removes the hotkeys), but it should fit your needs.
I believe you can find a possible answer at CocoaDev - DontExposeMe. Be very careful of using undocumented methods as such things can and do go away or change which may lead to unexpected behavior in your application.
Way back in 2004, someone asked this on Apple's Carbon Development mailing list and Eric Schlegel (works for Apple) responded. Basically, the news then wasn't good and suggested that this only be done for kiosk machines where you can set the system preferences up appropriately. He also mentioned this was a known bug - rdar://3499719.
Sadly, this does seem like something that you probably shouldn't try to do right now. I certainly wouldn't appreciate it if some application started messing with my system preferences as mentioned in the article Marcel pointed to and then proceeded to screw them up because it crashed or something.
So, I would suggest heading over to bugreport.apple.com and filing your own bug. It does make a difference because Apple does use the frequency of similar bug reports to help judge whether or not they should do or fix something. Make sure to include the url for the reply by Eric S. and the bug number he mentioned.