C.T.
E.-
I haven't logged in to read any of the other responses, but with my daughter, I would say "ouch" loudly and then tell her "No. You don't bite Mama." sternly. The ouch usually had a good startle factor to it. I would also couple this verbal response with a detachment (you know, where you slide a pinky in and break the seal). After a few moments, I would allow her to attempt to relatch. We would repeat as necessary and if she caused one side to be sore, we would switch and try again. She was a fierce eater and /hated/ this tremendously. She got the point very quickly.
One side note - I have yet to cross this bridge with my nearly 7 month old son. His teeth seem to be coming in much more slowly, but I imagine since it worked so well, I'll try the same thing with him too.
Good luck.
~C.