When the food starts to hit the floor, mealtime is over. I will remove the food remaining and they sit until we are all done. They also get a stern, "NO. Food does NOT go on the floor."
If I find that it's a big challenge, as in an all the time habit, I will bring it up with mom and dad to ensure that they are reprimanding at home. Sometimes parents don't think it's a big deal. But when we have 3 or more kids doing it, it becomes a big deal!!