A firm but gentle approach works best for me. For instance, if my tot throws her food on the floor, I'll gently remind her that food goes in the tummy and it's not nice to waste. Then, I'd offer her a smaller portion or a healthier snack instead, and explain that this is what she'll be eating since she doesn't want her meal. Sometimes, she realizes her mistake and asks for the meal back -- which is great, because that means she's learning! If she doesn't, well, it just means less work for me during clean-up.