Have a rotation of standard excuses you use, so you always have something to say, but they won't hear the same one over and over, and get in your face about it.
I had a big breakfast/lunch/dinner and am still full.
I'm saving room for dessert.
My stomach is a little off, so I'm resting.
I had just had some: it was delicious!
Last time I had that, there were...problems...so, no thanks.
Sure! Thanks! (discreetly drop off in the wastebin)
It looks so good, but I'm full. Can I take a piece for later?
I save treats like that for the weekend. (Thanks for the idea, Catharsis!)
I have had success with all of these. (Except the last, but I'll be trying it!)