IRILI'S PROFILE

Mockery. In 1993 50 Ku Klux Klansmen marched through Austin, Texas. Five thousand anti-Klan protestors turned up to jeer at them. Best of all, several hundred lined the parade route and mooned the Klan in waves. The media ate it up, and the Klan looked ridiculous. The hurt that they wanted to cause was met not with anger but with derision. The juvenile delinquents are just like the Klan in 1993: anonymous in their high-tech bedsheets, and threatening, but in fact, a minority. Let's use our superior numbers and metaphorically moon the boys who can't behave. They're social inadequates, immature losers. Let's tell them so, loud and clear, in front of their friends.
Shut them up. The right to speak in a public forum should be limited to those who don't abuse it. James Portnow suggested this one in his Extra Credits video on harassment. Anyone who persistently abuses others gets automatically muted to all players. The only players who can hear them are those who choose to unmute them. Or another of James' suggestions: New users don't even get the right to talk. They have to earn it, and they keep it only so long as they behave themselves. This means a player can't just create a new account to start spewing filth again if they've been auto-muted. Build these features into your games.
Take away their means. If you're the father of a boy who behaves like this online, make it abundantly clear to him that it is unmanly and unacceptable, then deny him the opportunity to do it further. We don't let nine-year-olds misuse tools to hurt other people. Take away his cell phone, his console and his computer. He can learn to behave like a man, or he can turn in his homework in longhand like a child.
Anonymity is a privilege, not a right. Anonymity is a double-edged sword. A limited number of people need it in certain circumstances: children, crime victims, whistleblowers, people discussing their medical conditions, political dissidents in repressive regimes. But those people normally don't misuse their anonymity to abuse others; they're protecting themselves from abuse. I think the default setting in all online forums that are not intended for people at risk should require real names. After a user has demonstrated that they are a grown-up, then offer them the privilege of using a pseudonym. And take it away forever if they misuse it. I haven't used a nickname for years except in one place where all the readers know who I am anyway. Has it made me more careful about what I say? You bet. Is that a good thing? Damn right it is.
Impose punishments that are genuinely painful. This suggestion is extreme, but I feel it's both viable and effective. To play subscription-based or pay-as-you-go ("free-to-play-but-not-really") games, most players need to register a credit card with the game's provider. Include a condition in the terms of service that entitles the provider to levy extra charges for bad behavior. Charge $5 for the first infraction and double it for each subsequent one. This isn't all that unusual; if you smoke in a non-smoking hotel room, you are typically subject to a whopping extra charge for being a jerk.