Ouch! I have found it best to absolutely not allow my nurslings to do things to me that I don't like. I have so far had two pretty aggressive nipple twiddlers, which, when they first started doing it wasn't unpleasant, but became annoying as they got older, around 18 months. Then it was hard to get them to stop. My advice is, gently but firmly say "No" and remove them from the breast, saying, "You have to nurse nice; pinching (or whatever) hurts mommy."
Right now I am dealing with my 2yo's near-constant desire to touch the breast he's nursing from. He leaves my other nipple alone, but the breast-in-use is sensitive too because I'm pregnant, and even just his hand on the side is painful. It's usually enough to break his suction and say, "Ouch." I have occasionally had to disconnect him entirely and put my shirt down. When he's serious about wanting to nurse, he's able to keep his hands down and nurse nice. :-)
When my oldest did this, I tried giving him a stuffed animal to hold while nursing; he threw it at me! My current toddler has been pretty willing to cuddle a lovey though.
Good luck Mamas. Don't forget that nursing is a relationship and you don't have to tolerate anything you don't like! :-)