"Stop cutting" will work for some, but not all.
I think Enki makes a good point regarding transexuals. Circumcision (if, and only if it were performed exclusively on adults who could decide for themselves), tatooing, and ear (and other) piercing are also examples of socially accepted forms of self-mutilation.
I certainly don't think any of these things ought to be illegal, provided the person having the mutilating done (by self or assisted) is an adult.
As I've stated before, I think this is mostly about garnering attention. This is because the fad has become particularly popular with youths. Please understand that I refer to it as a fad because twenty years ago, cutting was practically unheard of. The main reason people are doing it is because they hear about other people doing it.
I think the appeal lies in that it's a dark, scary thing to do. It's an inexpensive way of making a pseudo-martyr of the cutter - it helps to paint him or her as a "tortured soul", which - for a young person - can seem pretty cool (a la "The Crow", Marilyn Manson, etc).
Some self-mutilation is relatively harmless (as a kid, I used to bite off the skin around my fingernails). Whether it's seriously damaging or not, I think the decision ought to be left to the individual - unless he or she is a minor.
For minors, I think they need to be loved, but also kept from badly injuring themselves. I'd say, if they keep it up beyond one or two instances and it seems to be a serious health risk, they ought to be institutionalized.
I think this would actually aid the cutter in projecting the "tortured soul" image upon release. With good fortune, having stayed in an asylum like Hannibal Lector and other dark, tortured characters, will take the place of recreational self-mutilation.
Peacelove,
Aldousage