Is it just nipping or biting hard and hanging on?
The method I use now is to hold my finger in a ferrets mouth for a couple of seconds to make it gag. They usualy get the message in a few days.
I got a pair of albino jills years ago that hadnt been handled so were nippy. I sprayed my hands with bitter apple and every time they nipped I held my finger in the ferrets mouth for a second to let them get the taste. This put them off biting in no time.