^I'm a little confused about what ya mean about blocking all rewards?
I knew narcan would block opiates, but I never thought about your natural painkillers, so I'm assuming endorphins would be blocked too. It wouldn't help right off the bat with motivation to cut, but if you don't get a reward for doing so you wouldn't really have incentive to do it again. It wouldn't make them feel better, but it would (in theory) eliminate cutting as means of relief, so they would have to find another way to manage the stress.
I dunno if you could GET people to willingly take it, but I could see it maybe being useful in psyche centers if it worked.. And maybe if people in out paitent wanted the extra help while trying to stop.. in a way like that pill that makes you sick if you drink alcohol, but.. not really.. haha