Ferrets are armed with needle-sharp teeth. On average a dozen ferret owners will require medical treatment for bites. The key to changing a nippy ferret's behavior is to take away his motivation to bite. Your ferret bites because biting gets results – that scary, annoying hand goes away. But if biting brings no results at all, the ferret will soon stop biting. Remember to reward your ferret for proper behavior. When he's playing nicely, speak gently to him and give him treats.
For more information, please read the story How to Keep Your Ferret From Biting.