My favorite variation of this that really shuts people up: "If I eat that, I will regret it for weeks." Usually they understand the gravity of the situation at that point, but if they still push it, I go into a little bit of detail about my potential digestive distress and NO ONE wants to hear that
I also have a coworker that always brings lots of chocolate to work to share. He is really extreme about pushing it on people. I've given up fighting it; at this point I just thank him and take a piece. Then I either take it home to my husband or give it to a different coworker.
Edit: I also meant to say I am glad to see this thread is still kicking around, it is one of my favorites and I haven't been around for a while.