I did something similar with my kiddos (still doing with ds). As soon as it starts going on the floor, 'All Done'
Eventually I learned to anticipate when dd was finished and would scoop up the plate/bowl/etc and then praise her for not throwing food on the floor.
You get more of what you pay attention to!
The only other thing that I did with dd was *not* put her in the high chair. She had a travel booster seat that had a permanent place on one of our kitchen chairs. While she was eating, she was required to stay seated. But as soon as she was done, she would pick at her belt-strap and sign "Finished". I would let her down immediately to reinforce the desired behavior.
Again, you get more of what you pay attention to!
K. H, mama to