What works for me, most of the time, is to say "I don't wish
to eat that, thanks." For some reason, saying "I don't want it" in THAT way stops people from pushing me. I say it in a nice tone of voice, and I smile and all, but I emphasize the word 'wish'. And 99% of the time, the pushing stops.
And for the 1% who have become obnoxious, I ask them why
they're being obnoxious and pushing food on someone who has said
she doesn't want it, when my not taking some means there is more for every one else?! I deliver that with my best Old Lady Glare and frowny eyebrows, and funny enough, the dietary commentary just dries right up......
I descend from a strong maternal line of Glarers, so it comes naturally. Not female dogs, you understand, just autocratic, dictatorial, bossy, and possessed of a firm sense of their own moral rectitude. Which I don't entirely share, but I glare well.