Young ferrets are easier to train than older ones. Ferrets are very intelligent and learn quickly. Just because they are small, do not assume you should treat them like hamsters. Young ferrets should be discouraged to nip at human hands. They do not know that if they bite us too hard it hurts.
Ferrets have very thick skin and bite the living tar out of each other without much fuss.
This is part of their sibling play behavior. By biting you, they are really just playing. Young ferrets do not have the aggressive instincts that an adult may have. Do not beat the ferret. After a few times, the ferret usually gets the hint that biting your hand has its repercussions and will stop.