I have to agree that if kittens are taken away to young from their siblings, they don't learn how to "play" nice. I've had a couple like that and found out that if their playing got to rough when I was playing with them, I would then yell ouch however many times it took for them to stop. For one thing, the loud noise of yelling would be enough to make them stop and usually back off. They're adults now and we still play but if they get to rough, I yell ouch and they stop.