The advice I was given was to put them down, tell them firmly not to bite and walk away. It worked really well for me, only a couple of times I got bit, did it and it ended. They soon realise that doing it means no food. I know they dont realise they are doing it or they are trying out these new things they have but you have to let them know that its hurts and its not on. I had to do the same sort of thing with they playing around, if she was mucking around I put her down. Good luck.