Thats barbaric! Use nails. Just push them through the rabbit, so the points come out the otherside. No more biting down,
Actually, like lab-tastic said, hard mouth is hard to break. But, alot of retrieving drills might help. Also, having the dog carry around a dead rabbit on his walks. (out of the public eye) This worked for my wirehair. As he bites down, correct him. No guarantee's, but worth a try.
Good luck!