How do make a horse stop biting?

A good hard swift punch should do better than a slap. Biting is absolutely NOT acceptable. Fingers or more could end up in his mouth and... I spare you the gore from there. Don't hand feed mouthy horses. I helps the problem progress. The more he bites the more and harder you punch. Don't let him get head- shy though. Rub his head down, but don't let him bit or nip at you and get away with it. Biting gets harder to deal with over time. Stop it now or it will get worse.