Childhood biting and stopping it

My kids never really bit. But the little boy I babysit started to bite my daughter. We caught it right away and he quit pretty quickly. We would just give him a tap on the mouth and tell him no. He listens really well. He still bites once in awhile though. Most kids that age bite out of frustration because they can't talk to tell what they want. Usually when he bites my daughter its because they were fighting over a toy. Another thing you could do is to bite them back. Now i'm sure that will get people flipping out...but it works. You don't bite them too hard, just enough to get the point across that its not a nice feeling.