Originally posted by Yoda
When he does that , grab his mouth and hold it shut for a few seconds and tell him NO in a very stern voice.

My ex used to bite my dog back and she finally stopped doing it . Im not saying to do that though.
Please don't do this holding the dog's mouth shut routine! I find it on the abusive side and definitely not necessary.

An immediate and firm no with no physical contact and then a obedient position such as a firm sit or down (down preferrably) for a good minute or two should be adequate. If you are consistant when this occurs with the same response, your dog will realize this is not appropriate behavior. The key is your response and your consistent discipline.