You were probably directed to this web page because you were seen responding to a difficult person in a public forum, such as an email list, chat room, or blog comments. In a spirit of high-minded civility, you were giving them a fair hearing, which you thought might lead to a productive conversation. This is noble and laudable of you, but it will backfire. Responding to these people only encourages them. Each calm, reasoned reply you post merely stimulates another rant from them. The best thing you can do is stop responding right now.
The only way to escape is to break the cycle.
You may be tempted to try a more confrontational strategy first, responding to their every provocation with a flaming denunciation. This will be satisfying for a little while, but gradually you will realize it's like kicking a bag of wet sand: the person will come back with something totally unrelated, and you'll soon wonder why you even bothered. Meanwhile, others in the forum will see the exchange and perhaps follow up with flames of their own, which will do no good either. Don't waste your time or the group's time with this route.
The only solution is to stop the problem at its source — which is, ultimately, your attention. These people thrive on attention, on being responded to. They don't care what the response is, as long as they get a response. They don't see the content of what you wrote, all they see is the fact that you wrote something. Thus, the solution is to ignore them. Don't talk to them. Don't trade e-mail. Don't acknowledge their existence.
If you absolutely must respond, then follow up with a one-line pointer to this web page, so others understand what you're up to.
Together, we can win.