My three year old does this, and after I tell her twice I take her plate away and set it up on the counter for a minute or two. This usually upsets her and makes her think about it. It is really hard to break her though when other family members around her do it, and she even mentioned it how her Uncle (who is almost 16) eats like a cow (she didn't use that word though) with his mouth open, it is the most disgusting thing ever. It took several times for her to get the point and I still have to occasionally remind her, but if I have to say it more than twice the next time I just walk over and pick up her food. She has been doing much better lately and even makes it a point to tell me that she is chewing with her mouth closed.