smack them in the face with a brick. Just a thought, but probably the wrong one
I just tell them I'm allergic to it. That usually shuts them up. I have IBS and can't have alcohol, garlic, artificial sweeteners, caffiene, fried foods, or dairy. Telling them this helps. And if they pull the line, "Come on, just try a bite." I just tell them about the nasty digestive side effects of me eating that crap, lol
Good luck!