Cyber-bullying is one of the drawbacks of the internet. Most cyber-bullies are actually insecure, demure people in real life. The anonymity and lack of meaningful consequences for their words are what gives them the "courage" to act like knuckle-dragging fucktards and be abusive. This isn't really surprising considering these people probably either suffer from real life bullies or believe they do. As such, they take those frustrations out on people they disagree with over the net, where they are safe from actual consequences.
Then, there are the minority of these twits who actually need to be reported to law enforcement, as they really will go through with threats in real life.
Sadly, I don't think there's any meaningful means to stop these people. One can report them to police, but getting law enforcement to take these things seriously requires the capacity of the bully to carry out a physical threat. So basically, police don't really take cyber-bullying seriously if the victim isn't geographically accessible by the bully. All people can do it try to get accounts involved in cyber bullying deleted, or make friends with a really good hacker who could permanently rout the Bully's IP to the Disney site and block any attempt to get anywhere else...
Teens nowadays are beginning to become more careless about others and even possibly themselves. They seem to do it just to relieve stress, cyber-bully someone to the point of suicide, or simply for fun.
If something like this were to happen to you in small places like South Korea, chances are, your whole life is destroyed within days since rumors spread far faster in much smaller countries.
But anyways the only thing I can think of to stop cyber-bullying is to give an OPPOSITE response of what they expected, or say something that will defeat the purpose of why they were cyber-bullying him/her.