K.P.
Consistency is the most important thing. I work with a couple of early-childhood education experts, and they are appalled when parents suggest you hit, pinch, scratch, or bite your child back. When you do that you reinforce the idea that biting, scratching, hitting, etc are valid ways to solve problems. Their suggestion is to watch her behavior and figure out when she does it- is she tired? does she feel neglected? is she angry? Frustrated? Once you figure that out, try to "head it off at the pass", when you see that she is getting to the point that she will bite in a couple minutes, change the circumstances.
Also, discipline has to be consistant. Give one warning, not six. And this is for everything, not just biting. I think with biting, there is no warning, you say, "You may not bite" and then you remove her from whatever fun thing you are doing. If that means going home in the middle of a shopping trip, so be it. I know that sometimes I am tempted to let something slide after I have given a warning because I don't want to stop being outside, or the walk, or the fun craft- but if I do that, then my son knows that Mama doesn't always mean what she says. Try to keep her frustration to a minimum- part of that is just saying, "Does that make you mad? Do you want Mommy to pay attention to you?" Part of that is giving her the words to use when she is angry or frustrated, so that she doesn't have to resort to pinching. Try to describe what she is frustrated about- it will help.
So, in conclusion :) Be consistant, don't react in a big way when she pinches- say, "You may not pinch. Now we have to go home." and then march her back to the car- no more conversation, if she continues to pinch- just keep on going back. Later you can talk to her about it, but that may not work until she is older.