This is one of the most troublesome areas for ferrets owners. Ferrets are often unjustly accused of being biters and I don't want to give the impression that ferrets are terrible biters. However, nipping in play is a natural behavior for ferrets, as it is for puppies and kittens. In nature, an animal's parents and peers teach them not to bite, and as pet owners we assume that responsibility.
It is best to start this training as soon as possible, as once it becomes habit it is harder to break. The simplest method is to withdraw attention when nipping occurs. A loud startled "hey" or "ow" combined with pulling away should convey that nipping hurts and you do not like it (although try to refrain from using to high a pitch as this might excite the ferret further). If the ferret persists in nipping, a short break in the action by turning away or walking away can help get across to the ferret that nipping is unacceptable and you will not play if the nipping continues. If this still doesn't work, a time out in the cage or some other confinement may be necessary, more as a chance to give the ferret a chance to calm down as an excited ferret will be more likely to nip. A stoppage in play is sometimes the worst punishment as far as a ferret is concerned!
In all of these cases the basic principle is a withdrawal of attention and playtime when an undesirable behavior occurs. It will also help to praise and give lots of positive attention for playing without nipping. Show the ferret that not nipping gets it much more attention than if it does nip.
For many ferrets this will be enough, but some are tougher cases. Many owners advocate scruffing the ferret (holding by the loose skin on the back of the neck, often with some gentle shaking or pulling, which approximates what a mom ferret might do), or gently pinning the ferret down until it calms down, among other more punitive methods. Of these, scruffing is most natural to the ferret (since that is how it would learn form another ferret) and some argue the most effective. Use of these techniques can backfire with some ferrets though, especially those who react very strongly and poorly to physical restraint. For further information, the Ferret FAQ from Ferret Central has a section on nip training.
Of course, ferrets also nip or bite for any number of other reasons besides play. If you take in an older ferret you may inherit some past problems that motivate the ferret to bite - for example, out of fear. Regina Harrison's Problem Ferret page discusses in detail some of the various reasons ferrets may bite and how to handle them. If you are dealing with an abused or neglected ferret, any sort of physical punishment is likely to simple make such a ferret more fearful and distrustful. For such cases Regina Harrison's page will help, especially with understanding why a ferret is biting and what can be done to try and correct it. Regardless of the cause, persistence, patience and consistency will be necessary - the nipping will not stop overnight.
Beyond this, ferrets can learn some simple tricks and also how to walk on a leash. Applying the principles discussed above - i.e. rewarding the behavior you are looking for, along with having patience, should make your ferret training endeavors successful.
