The last time someone did this to me, I was biting back the reply, "It would be better for me to hit myself over the head with a hammer than eat this!"
But, I didn't. Just smiled and said no thanks, and the person finally remembered I was on a diet.
I like the "I'll eat it later" response, though. I may try that next time, if it isn't ice cream "cake" again. I can feed it to my chickens and turn it into an egg!
w00t!