Ferrets are very robust little animals but can also easily injure themselves. I've read about a ferret falling from a 2nd or 3rd story window and surviving.  Then again my mother told me when she was young a weasel or polecat (very similar to the ferret) snuck into her house and broke its neck while climbing up to eat their birds. So let's outline what is acceptable, and isn't acceptable for play.

Acceptable Roughhousing Ferrets love to play rough. It's totally fine to roll them over on their backs, tickle their tummies, just wrestle with them. Amongst each other they play very rough. Of course keep in mind that they are small animals and you look like a giant to them so don't overdo it. Scruffing If a ferret gets out-of-hand and starts nipping hard or biting you should pick them up by their scruff. This is not painful for the ferret, in this sense they are like cats who can always be carried by their scruff as well. After scruffing you should proceed to discipline them. The best method in my opinion is to hiss at them so they understand you're upset, tell them "NO", then put them in time-out for 5 minutes. Licking I would never punish my ferrets for licking me, just be careful because a lot of times a sweet little ferret kiss will turn into a not-so-sweet little ferret nip. Very Slight Nips If you use your hands to play with your ferrets expect them to mouth you some.  If however this hurts you then they are being too rough.  Mine will often put their mouths around my fingers, but not bite down.  I don't have a problem with this. If you are uncomfortable try using a toy to tease them with instead of your hands, that way they can chomp down to their heart's delight. Just remember to a ferret their mouth is like their hands.

Unacceptable Hitting Never, ever hit your little ferret, no matter how hard he/she bites. Remember how large and intimidating you look to them and how painful just a slight hit from you would feel. This will only encourage your ferret to be afraid of you and to bite your hands. You will have a traumatized ferret if you do this. Throwing Although they can be shaped like a boomerang at times, playing catch with any living animal is wrong. My cousin did this with a hamster once when he was very young and it had a heart attack. This doesn't mean tossing a ferret up 6 inches onto a soft couch. Kicking Ferrets have foot fetishes and will often attack feet. Please don't kick your ferret to get him/her off. Gently shaking is ok, try using your hands to gently dislodge them. Then discipline them as I have written above. Bites This is a no-no on the part of the ferret. Do not let a ferret get away with biting you. Always discipline them. Especially do this if they are kits or might be much harder later on. By biting I mean hard nips that leave teeth marks, or any bite that draws blood. By teaching your ferret not to do this you may be saving its life as well considering if it bites someone else there may be a big legal mess resulting in its death because of it. Pulling the Tail Ferrets are not rats that you can lift by the base of the tail. This is painful to a ferret. Their tails are not handles. Please never pull a ferret by its tail unless its on a slippery floor of some kind where that really wouldn't have a painful effect, or incase of emergency, like having to grab the ferret before it falls to its imminent doom, etc.