Whenever dieting, when someone at work would offer me something like that my standard response was "You fucking know I can't eat shit like that...asshole/bitch (depending on gender)! How about if I stick that brownie up your ass!"
Nonerz, try it, after you do that to about 4 or 5 people in front of a group, especially when no one at work has ever seen you not be cheerful (I'm happy all the time in public), NO ONE will offer you junk food again.