With sarcasm, say to them - "Sarcasm is the domain of losers."
If standing in front of you - "Do you know what I see. I see 112 pounds (or whatever) of I don't give a crap." And then walk pass them.
After a negative comment, look at them and say - "Sorry, I can't understand you. I don't speak dumb-butt."
And then there is always - "you've obviously mistaken me for someone who gives a crap."
All of the above will surely antagonize this person and maybe make it worse. I am guessing that this person has issues in there life (family, friends, school, jealousy) that causes them to only be able to feel good by thinking they are making someone else miserable. Consider trying to understand where they might be coming from and maybe find a way to empathize with them to convert them, not necessarily into a friend, but at least into an acquaintance.
If all else fails, do whatever you can to avoid them and if the situation escalates, get others involved, i.e. parents, other adults, school officials, law enforcement, etc. Don't let it escalate into physical violence.
I probably should have asked first how old you are. But in any case, don't lose your cool, don't raise your voice or let them see you get upset. Keep your confidence, self-respect and always remain controlled, even smiling. Remember, they only are satisfied when they can get to you. So, don't let them get to you.