I will tolerate a LITTLE bit of biting, when it is age appropriate. A biting 4-year old goes out the door. A biting 16 month old will get some chances.
As a provider, part of my job is to PREVENT bites. It's tough. I can't ALWAYS do it. But, once I know I have a biter, I keep them VERY close for several weeks after a bite occurs. I discuss with both parents, let them know that I have a plan, give suggestions to the biter's parents on dealing with it, and let them know that I can not allow it to continue.
The biter WILL stop biting (someday
). If it is down to a parent being willing to pull over it, then it is probably time for you to make a decision about which family you would rather keep. If the biter is targeting the same child each time, the situation could be resolved by EITHER child leaving.
I would weigh which family I want to keep based on their merits, and proceed from there. If the biter's family is a better fit, you can try to convince the other family to give it time (don't be surprised if they don't, and don't be surprised if they bad mouth you after you let them walk away, either). If the other family is the one you want to keep, simply tell the biter's parents that you can no longer keep their little barracuda because the biting is endangering another child.
Sometimes, you can work it out by discussing your plan with BOTH parents, and keeping the kids separated as much as possible, as well as working with modeling good behaviors. Here's a book from a series that I LOVE:
http://www.amazon.com/Teeth-Biting-B...not+for+biting that really seems to hit home with the little ones and help turn behaviors around.