Deaf Ferrets may nip hard because as a kit when brothers and sisters squealed "arh! your hurting me" they didnt hear them so they missed that important lesson in a ferrets life , that ferret teeth are sharp!
but with time and patience you can teach them that biting is not the way to play with humans, scowly faces and rumbled No's work, always remember they can see facial expressions and feel vibrations.
Oh and between the nips there are always the kisses. :) ~ Shewolf