I agree with Nadine's suggestions about time out.

One of my DD's carers said Katy would bite when she saw the other children doing it, so she thought it was acceptable behaviour. I guess as long as the children are made aware that it is unacceptable behaviour, there's not much else you can do.

I have also had times when she bites herself when she is frustrated.

Tell them that is it not nice to bite their friends, and let them know how it makes you feel when you are hurt.

Good luck with it.