
It takes time - basically dogs have to learn
not to try to find out about the world that way. They don't have hands to explore with, after all - it's a bit like teaching a toddler 'don't touch!'
What I've found works with mine is to say calmly 'Ah ah, gently' when they get a bit hard, and not allowing games to become too exciting. The calmer and quieter his environment, the less likely he is to get wound up. Give him a few more weeks and you'll see a difference. :)