Say you don't wanna, and give money for the cause instead. Even for guys, moustaches are gross, anyway. If you need an excuse, say your girlfriend/wife/roommate/sister/mother absolutely refused that you do such a thing. Most women wouldn't accept the participation of a man close to them in Movember without a frown anyway.
Oh yeah, as for excuses, there's always saying that you get awful ingrowns if your beard grows. It's true for many people.