They went to school together, it didn't just happen online.It's effective, if ignored people simply leave you alone on the Internet if they cannot get any reaction from you, they feed from your distress, if you simply don't give a flying fuck, then they seek another victim to bother. Blocking/Ignoring is a very effective way to deal with online bullies. Fighting back might work too, but I would advise against it, just simply never-ever react and they will surely leave you alone since you are not an interesting target.
This doesn't work for real life however where you cannot hide what you actually feel and where people can actually follow you and bother you on the strangest places. Punching is dumb advice, most kids that get bullied are weak, if they were the strong type others wouldn't try bullying them. Paying somebody to beat another is yet another stupid piece of advice, anything that could cause retaliation is stupid. Best IMO is to tell your parents and even the police.
But online? Nope, don't get triggered, don't respond, don't argue, don't try to communicate in any way, just close all paths to you down and mind your own stuff.