Hi
I can completly understand your problem, when we got bailey our choc lab, I thought at one point that i had brought the only nasty lab pup to be born ever!!! She was like a crocodile sometimes, i looked on all the forums and everyone seemed to say the same thing. The loud ouch and walking away worked, sometimes!! other times it just made her more excited. I have to say the thing that helped the most was when she went to doggy day care, the other dogs soon taught her what was too hard. She pretty much stopped nipping in a week. She is 5 months now and very soft in the mouth. Thank goodness, my hands and arms looked dreadful. It will end, you just have to be really strict with saying no and moving away from them.

Goodluck