Only now he's grown into an adult dog and those play bites aren't quite so funny. In fact, they hurt. Clearly, he isn't being malicious, the tail wags and he seems generally delighted by the game, but you don't find it amusing anymore. You've tried pulling your hand away, shouting, and even resorted to smacking, but all that happens is he gets more excited and ramps up the fun.
What to do?
Ideally, start by teaching bite inhibition to a puppy. This can require you to use techniques such as timeout out or redirecting play onto a toy. An adult dog, with the potential to do harm, is a more serious prospect to deal with. Consider the suggestions below, but also know that for safety's sake it may be necessary to consult a certified behaviorist and have a safe strategy put in place.