LOL Michelle, it's frustrating isn't it. Jacob used to do this all the time.

A few things we did to try and rectify this were:

~ Getting rid of the highchair and letting him sit at the table like a big boy to eat (he either sits at the dining table with us or sits at his own little table/chair setting).

~ As soon as food throwing/pitching it onto the floor began, we immediately removed the food. Our theory was that if he was throwing it about, he clearly wasn't hungry/interested enough to be eating it. So we would remove it (with as minimal fuss as you can manage LOL). He soon worked out that once he did it, it was gone.....

HTH? Good luck.....I know it's a frustrating thing (perhaps a boy thing?!)