Ah, the tactical eating game. Now, take whatever is being pushed. (From those that get offended if you say no) Hold it for ages, maybe nibble, put it down. Or you're no longer hungry. You probably do this already without noticing. Just make it a conscious polite politician type of game. "Well, I tried."  Try the merry go round, palm it off to someone more gluttonous, as soon as they finish. Also, deflect the conversation back to what they will eat next, make them think about something bigger.