We tried to teach DS how to move his food to one corner if he didn't want it, so he didn't have to throw it overboard. So after he threw it off, I would pick it up and put it in the corner of his tray and tell him that if he doesn't want it he should put it here.
You could also startto take her out of her highchair when she starts throwing food, to let her know this isn't appropriate eating behavior.