No, thank you, I'm full/ I've had enough/ I couldn't eat another bite.

You mean when somebody is hosting a dinner and urging you to eat more? Then it depends on the atmosphere! But you can simply say, "No I think I'm good, thank you." Or if you want to sound even more enthusiastic you can also say something like, "Oh that's lovely of you but I think I'm okay/good, thank you very much". Or, if they made the food, you can also add a brief compliment on the food, like "I think I'm okay now, thank you very much, but that was very delicious." There are many ways to politely decline the offer but these are just some ideas!