A Cyberbully problemo.

Contact police.

Reveal his secret identity to Batman!

Not much you can do expect the obvious thing..ignore the guy. It's not a difficult thing to do. And what do you mean by exploting personal info to humiliate people?, he makes fun of a picture on facebook?

POLICE. If he is bullying you then, don't use internet.

Rape him in dark alley

Avoid internet for several months. That would make him look for another target. If you aren't in the net you can't be cyberbulled

I don't know what I would do, but I'd try something and see if it helps, like: change your privacy settings (in every social network and the likes) to hide as much as you can; use VPNs yourself so he can not track your activity while you're online; ask your friends to not tag photos of you and don't do it yourself; totally ignore any attempt to contact you and their messages; don't accept 'friend requests' from contacts you're not sure about, and if it looks like it's coming from a legitimate friend, double check with them by other means that it's actually them trying to get in touch; avoid posting much at all...

The way I see it, people who do these kind of things are or have been victim themselves of the same kind of harassment, often within their own family, so they are to be pitied in the first place :( No need to let them ruin your life

If he does what you described then "giant dick" is a huge understatement. I'd say he's a criminal.

Also people saying "if he's bullying you then don't use the Internet" are ridiculous. So if I'm getting prank calls I'm supposed to shut my phone off and not use it? What if someone harasses me where I live, I'm supposed to move out? I'm not being harassed, just laughing at your logic.

This empowers such dicks and makes them think they have huge power over their victims. I think the best way is to either ignore them or stand up to them and be firm.

What exactly does he do? And how does he exploit those private informations?

Only way to stop him is to kill him. Do it. You will be a champion among peasants.

Gather as much evidence as possible.
Report to police.

right click , ignore/mute, done.

You need to totally disengage.

Stop reading his posts, stop refuting what he says, stop replying to him, stop mentioning him, stop answering anyone who brings up anything that he says. Don't tell him you're ignoring him, don't tell other people to ignore him, don't refer to him, don't refer to anything said by him, and don't refer to anything that anyone else says about him. Let him lie his ass off. Let him tell the truth. Let him do whatever he hell he wants and don't give him the slightest acknowledgement he exists.

And I don't mean for a day. Or a week. Permanently. Commit to it. Any time you catch yourself typing his name or anything about him, close it out completely. He only has as much as you give him to work with, and if you give him literally nothing, he'll eventually decide it's a waste of his time. It will certainly take more than a month. It may take a year. But unless he comes to you in real life, he won't be able to do anything to you for as long as you're disengaged.

And if he does take it to real life, you now have physical evidence to give to the police. But given that he's in another country, that seems unlikely.

EDIT: I should emphasize that this also means not reporting him to any site admins. If the stuff he's posting is as bad as you say, other users will report it for you. And if it's not that bad, there's no need to worry about it. The more effort you put into stopping this guy, the more he's succeeded. If you go to zero effort, then he's wasting all of his.

How did you even get into this situation? From how you met him, what you did to piss him off, and what he's threatening you with exactly. It's very strange to be harassed as much as you're saying from a random user that targets people out of boredom. And what is the situation with the other users he's harassing?

OP, try putting "report a cyber bully" in Google. I did and there are some useful articles there.

How can someone harass you online if you just dont give a f*ck?

You should deal with him the same way you should deal with a real bully. Just don't give a shit. Bullies thrive off of attention and this thread proves that your attention is very firmly fixated on him. Stop giving him the attention he wants and he won't have anything to gain. As for stopping him from bullying others: there isn't much to be done. Maybe pass the same advice on to them.

If he hides behind proxies, then how can you be so sure he lives in Greece?

Can't you just block the d*ck, and don't give a f**k about him?
and how does he know personal info about you if he doesn't live in your country/ not your relative...

Just ignore him and ask people to do the same.

If i understand correctly(based on the country you have on your steam profile) you yourself do not live in grece so what exactly can he do to you? I understand he can make threats but it's not like he's gonna get on a plane and come and do something to you. Also what can he do to you online? This is the part i don't get, do you actually know he has something on you (pictures/video/whatever) or are you just afraid he might have?

Like others have said, simply ignore him and he will go away. Even with a thread like this you would only feed his idiotic behavior not scare him away. I don't know how police works in your country but in my crappy country if you went to them because you're getting cyberbullied they'd laugh at you and send you home. Unless there's blackmail involved, aka him trying to force someone else to do something for him, i doubt police anywhere would care.

Just look at all the cowards that think they're experts on the topic... "Just ignore him, just block him." There is a fine line between a troll and someone who goes out of their way to contact you again with death threats, this isn't a "ignore him" type of situation.

Wait, you know him in real life?

Just get his address and name and phone number and give them to his victims. Then walk away and call the cops.