Uh... so what is going wrong in our house? DD will be 3 next month and STILL does this sometimes. It drives me bonkers, but I've developed lightning fast reflexes in my attempts to keep as few things from hitting the floor as possible. She learned the sign for "all done" when she was about 14 months old, but could say "all done" when she was 15 months old and stopped using signs when she started talking, so there was never any problem for her to communicate that she didn't want to eat anymore. Ugh.
lmk1, we took kind of an opposite approach--when dd starts throwing, it's clear that she wants out of this whole meal thing and is anxious to get out of her seat. So we don't react to her other than by moving everything out of her reach, finish our meals, and then move her chair over (with her still in it) so we can clean up the floor. So throwing stuff doesn't equal escape from dinner; she still has to sit with us while we all finish our meals. But she still does it sometimes, so I just don't know. I'll keep waiting for her to outgrow it, I guess.