When I bred bunnies I got the occasional nip. I found that the water worked well, as well as a GENTLE thump on the nose with two fingers. You aren't trying to make the bunny feel it, just get his attention. Those are the only methods I have tried with any success, but then again I don't remember trying anything else. I'll be trying a few of the other suggestions on a 'nibbler' bunny at my work.