When food starts getting thrown and a simple instruction not to is ignored, then meal time is over. That's how we did it with my little one. It only happened a couple of times. Simple. No stress for you and it gives her some control over deciding when she's finished. It's not fun if you can't throw it anymore, so it stops happening.

FWIW though, our DD was more worried about whether the dogs would eat her dinner if it got chucked over the side, so as long as she was still hungry it never went over!