[FireHeart]

All the suggestions listed above are great and should work effectivly. Just sit down and talk to them, let them know how you feel and educate them on what self-harm actually is and why people do it. They need to realize that it's not a joking matter. However, do remember that they say the things that they say out of ignorance, don't come on to them blaming them because then they woun't listen they will just try to defend themselves.
I wish you the best of luck <3