We all know hosts, friends or family, who aren't satisfied until they cajole us, beg us, guilt us, to eat more, more, more. Their entreaties are hard to resist, if only because we want to be polite.
Honestly, I don't think most of these people are bad people who are trying to undermine our efforts to lose weight (although some of them, regrettably, are). I think they're just people who 1) want us to enjoy the party with everyone else, and 2) are a little dense.
If you say, "I can't, I'm on a diet," that will only encourage those who want you to 'enjoy the party,' because what they're hearing is, "I really want the food but I can't have it." Which is basically true, but they usually set out to change your mind about it when you really want to stand firm. And standing firm is hard enough, even without someone urging us to eat.
Some dieters find that telling the host you're allergic to an ingredient will stop the pressure, although that won't work with family who most likely know what you may or may not be allergic to. The best response is a firm "No thank you." If the host persists, an "I'm so full I just wouldn't enjoy it right now. Maybe later," may do the job. If the pressure continues, find an escape -- take your plate to the kitchen, engage another guest in an intense conversation, or, as a last resort (and if the food pusher is too obnoxious to bear), find a reason to leave.
Before you go to your next occasion, take a few moments to reflect on the last time you felt pressured to eat by a food pusher. Try to remember that perfect response you came up with after you had left the party. Work out a few ways to say "thanks but no thanks." Then practice in the mirror so you can be prepared and stand your ground the next time someone tries to push food on you.