Dr. Judith Beck, a leading author on diet and motivation, says most food pushers aren't trying to sabotage you but are trying to please you. Some food pushers though feel guilty about their own eating and want you to join them so they feel better.
When someone is trying to push food on you, Dr. Beck recommends you just say no thank you without any explanation. Say it over and over. They'll get the message you're not going to take the food and you're still being polite. If you say no thanks, I'm on a diet, you're sending a double message. You're saying, I'd like it but I can't have it. Odds are they'll keep pushing until you give in. Just say no thank you without any explanation.