First off, there's no freedom-of-speech or censorship issue involved in moderating or deleting comments. This is my space, period. Freedom of speech gives a person the right to put a political bumper sticker on his car. It doesn't give him the right to put that same bumper sticker on my car, not unless I choose to let him. It's amazing how many people don't grasp this point.
Not everybody who disagrees with me is a troll. I almost never reject a comment just because of disagreement. It's when somebody adopts a rude or hectoring or superior tone, or a particular person never comments except when they have something to criticize -- well, it's obvious when you see it. I know that rudeness is pervasive on the internet, to the extent of being accepted by many people as part of the norm. I don't care. I won't put up with it here.
The comment moderation originally was, and still is, necessary because of the volume of spam "comments" I get. If it weren't for moderation, every post's comment section would turn into a thicket of ads for everything from viagra to designer handbags to Japanese porn sites. The only reason you don't see that stuff is that I see it first.
One increasingly-common type of troll, what I call the "change the subject" troll, is actually pretty similar to a spammer. Such a troll starts with "This is just an attempt to distract from....." or "This is ignoring....." or similar, then proceeds to tell me what he thinks I should be writing about instead of what I actually did write about. Sorry, no. If he wants to discuss something else, even designer handbags, fine -- but on his own blog.
All this being said, again, it's actually pretty rare that I delete a comment from a real person. But I've seen what happens to bloggers who let trolls comment freely. Once allowed to take root, they settle in and multiply, and comment threads turn into an interminable, exhausting mess of back-and-forth bickering. In the case of right-wing political trolls, it's been credibly claimed that this is a conscious strategy.
As for the real psychos, the ones Cristina Rad (amusing video below) calls "haters" -- the kind of people who develop a weird obsession with you and harass you for, in some cases, weeks or months -- I've had at least two of those in the six years since I started this blog. (It might actually have been more -- over time memories of these people kind of run together in my mind.) One was a Christian fanatic, the other was an atheist with serious mental issues.
What I've learned from experience is that trying to reason with psychos does not work. Trying to debate them does not work. Telling them to go away and leave you alone does not work. Their whole purpose is to get a rise out of you. If you give them any sort of reaction at all, especially if you're spending substantial time and energy on them and/or getting angry and frustrated, you're feeding them. And they will not go away.
So I give them nothing. I delete their comments in moderation, I don't reply to them, I don't answer their e-mails, I don't write cryptic-sounding posts obliquely referring to them, I don't give them any response at all. Once I get their schtick, I don't even bother to read their comments before deleting. Eventually, un-fed, they go away.
I've seen what happens on other blogs when bloggers don't get this. I've seen bloggers write paragraphs meticulously correcting some troll's distortions of what the blogger said (the Christian psycho mentioned above was quite good at deliberately twisting things I'd said in hopes of goading me into replying to correct him). I've seen them beg and plead with trolls to stop commenting, while I'm rolling my eyes and thinking "just delete the damn comments and save your time and energy". I've seen them responding with debate or with anger or with ridicule or with whatever, but still responding. And the troll never goes away. He knows he's getting under their skin. He's being fed.
Predictable as the pattern is, it's not normal behavior. If any blogger were ever to simply tell me that my comments weren't welcome, I would immediately stop commenting there. Why not? There's more interesting stuff on the internet than I could ever possibly have time to read. Why waste time on a blog where I'm not wanted? But the hard-core, obsessive troll is a mental parasite. He gets his jollies from inflaming you. I don't play that game.