How Do You Handle Food Pushers?

Food pushers are people who insist you MUST eat a certain food. They do not accept "No, thank you.". They do not even accept medical reasons.
Example: "Aunt Martha baked it from our special family recipe and she'll be hurt if you don't have a piece."
Another typical one, "It's Lillian's birthday and we're all having a piece. I'm not going to argue with you, I'm putting it right on your plate."
And the protest against cutting calories: "No!!! We're Italian! (Or Hungarian, or German, or Roumanian) No one diets at our house!"
When I told my cousin I was following my doctor's advice, she looked around the room and said, "Does anyone here see her doctor? Because I don't. He's not here, so you can eat it!".

Please don't tell me, "I just say a firm No, thank you!" Because with genuine food pushers, normal responses like that don't work. They see it as a contest they're determined to win.