Often times, you will hear people refer to ferrets as "biters" or "vicious". People who say these things usually have never been around ferrets or have had a past negative experience with a poorly socialized ferret. Ferrets use their teeth to communicate. Since they can't speak and don't have hands, they use their mouth to explore the world and communicate with other ferrets, animals, and humans. Baby ferrets will go through a "nippy" stage as do most baby animals. The nipping will start to diminish at around 6 months of age or older. Not with all ferrets though. Some have to be taught not to play like that. If you've ever watched two ferrets playing together, it looks pretty rough. They look like they are really hurting one another, though they aren't. They will wrestle and bite each other, provoking play.
Ferrets have a rather thick, leather-like skin, so another ferret biting them doesn't hurt. However, a nip or bite of the same magnitude on a human's skin definitely hurts. They don't realize our skin is not as tough as theirs, and they play with us as they would another ferret. So we, as their owners, have to teach them what is acceptable play behavior and what isn't. Because ferrets can be rather stubborn, any behavioral training should be very consistent. You should reprimand them IMMEDIATELY after they've bitten. If you wait too long, they won't have any idea what they did wrong. This way, they connect biting with something negative. You can also encourage positive behavior, such as playing gently and not nipping, using treats and praise as rewards. A certain amount of "mouthing" during play is ok, as long as they aren't hurting you.
There are several different techniques that can be used to train a ferret not to nip or bite. First it's good to know the difference between a "nip" and a "bite". A "nip" is basically the ferret "mouthing" you; putting it's teeth on your for brief moments with some pressure, occasionally pinching your skin. A "bite" however, is usually a lot harder. And depending on how hard the ferret is biting, skin may get broken and blood drawn. Even if the skin isn't broken, a "bite" will still leave marks. Some ferrets who bite will grab on with their teeth and not let go. This can become a long term problem if not corrected when they are fairly young. A signal that a ferret is going to bite you is when it brings it's whiskers forward.
Not every technique will work on all ferrets. You have to find the one which gets the point across best to your ferret. One method is to scruff your ferret immediately after he/she bites you and yell "NO!!". Unfortunately this doesn't work on ALL ferrets as most of them have no idea what the word no means. For some just yelling the word no or "ouch" is enough to stop them. Another idea is to put your hand over the ferret's face and say no loudly as they are going to bite you. Gently pushing the ferret's mouth away from you while saying no can work as well. Be careful with this last one, though. Pushing the ferret away may make him think you are playing and may only encourage the biting more.
Another method to try is to scruff the ferret, then drag it back and forth across the floor (while saying no loudly). This is the way mother ferrets discipline their young. Be careful though, when doing this, you don't want to hurt the ferret. Time-outs is another tactic. This is where the ferret has his play time cut short and is put somewhere away from his play area, such as a cage, pet carrier or other area, by himself. Similar to when your parents sent you to your room when you were bad. Though this can be an effective method, there can be some drawbacks. If you are using their cage or pet carrier as their "time out" area, they will constantly think they are being punished every time they are put in them; even when they aren't actually being punished. So if you are going to use this method, choose a neutral area--maybe a cage that is only specifically for time outs, or maybe your bathroom (with nothing on the floor with him to play with, of course).
You can also bend your finger and shove your knuckle to the back of the ferret's mouth and say no loudly. This makes him uncomfortable and may make him think twice about biting again. Another option is to put a handful of coins in a coffee can. Whenever your ferret goes to bite you, shake the can quickly. This will probably startle him, but will make him think twice about biting you again. One method I have used with a great deal of success is with a product called Bitter Apple Furniture Cream. It is non-toxic and gives the ferret a bad taste in it's mouth. It's mostly used to keep pets from chewing on furniture, wires, etc. Many people use the Bitter Apple Spray. They spray it on their hands so that when the ferret is playing and bites them, they get a bad taste in their mouth. Unfortunately, the spray tends to disipate quickly and you have to keep re-applying it. This for some reason, conditions the ferret to the odor of the spray. I've had some who would wait until the smell disipated, then bite me; when my hands smelled of it, they wouldn't bite me. One ferret in particular would pull his tongue back when he went to bite me so he'd never actually taste the bitter apple on my hands. When I use the bitter apple cream, however, I don't put it all over my hands. What I do is when a ferret bites me, I immediately scruff them, put the dispenser tip of the bottle in the corner of their mouth, and pump one drop in their mouth. I do this consistently, each and every time they bite me.
Though it doesn't happen often, some ferrets may bite and not let go. There are several ways to make them release their jaw from you. One thing is to take your index finger and thumb and place one on each side of the ferret's jaw. Place the fingers on the joints of the jaw and apply pressure (by gently squeezing). Another thing to do for this type of bite is to block the ferret's nostrils with two fingers (one over each). Since ferrets always breathe through their nose and not their mouths, this will make him/her open it's mouth to breathe, thus releasing the bite. You can also run the ferret's head under faucet water--it'll startle them and release the bite. Even though this type of bite does not occur often, it's still a good thing to know how to resolve the problem just in case it does occur.
The key element to training your ferret not to bite is consistency. Stick with only one method at a time. If you use a different method every time you discipline them, you'll only confuse them. Use one method for a week or so. If it isn't working, then try another until you find one that works. You'll know you're getting through to your ferret when he puts his ears back and squints his eyes. Another important thing to remember is never hit or kick your ferret to discipline it. Not only will you hurt it, but you will also alienate it and possibly make him start biting out of fear.
These are just a few of the methods that have worked for people trying to break their ferret of the biting habit.