If someone is really pressuring you to take food you have already politely declined, ask them
why they want you to have it so much. A lot of times it's because they just want someone else to be eating the same thing that they are, to lessen their own guilt!
Personally, I have NO problem being stubborn on this issue. The more they try to get me to take it, the more firmly I dig my heels in. If I need to, I let my annoyance with their pestering start to show.
After all, would you be polite and take poison from someone just because you want to avoid being unpleasant about it? Maybe that's a little dramatic, but that's how I see it.
Anyway, if you are really unable to say "no" to people, then I have two solutions for you:
1. Lie. Say you are allergic to chocolate/gluten/wheat/eggs/lactos or whatever ingredient is in the food you are being offered.
2. Accept the food, then throw it out or give it away later (without letting the person who gave it to you know, of course). Tell the person who gave you the food that you are really looking forward to having some, but can't right now because you just ate and are too full/you have a tummy ache/you'd like to save it for later, or whatever excuse that comes to mind. In short, see rule #1.