If you are new to online communities, pay close attention. If you are a moderator or "owner", internalize this advice ... if you want the community to thrive and prosper.
Good online communities die primarily by refusing to defend themselves.
Somewhere in the vastness of the Internet, it is happening even now. It was once a well-kept garden of intelligent discussion, where knowledgeable and interested folk came, attracted by the high quality of speech they saw ongoing. But into this garden comes a fool, and the level of discussion drops a little—or more than a little, if the fool is very prolific in their posting. (It is worse if the fool is just articulate enough that the former inhabitants of the garden feel obliged to respond, and correct misapprehensions—for then the fool dominates conversations.)
So the garden is tainted now, and it is less fun to play in; the old inhabitants, already invested there, will stay, but they are that much less likely to attract new blood. Or if there are new members, their quality also has gone down.
Then another fool joins, and the two fools begin talking to each other, and at that point some of the old members, those with the highest standards and the best opportunities elsewhere, leave...
In the beginning, while the community is still thriving, censorship seems like a terrible and unnecessary imposition. Things are still going fine. It's just one fool, and if we can't tolerate just one fool, well, we must not be very tolerant. Perhaps the fool will give up and go away, without any need of censorship. And if the whole community has become just that much less fun to be a part of... mere fun doesn't seem like a good justification for (gasp!) censorship, any more than disliking someone's looks seems like a good reason to punch them in the nose.
(But joining a community is a strictly voluntary process, and if prospective new members don't like your looks, they won't join in the first place.)
And after all—who will be the censor? Who can possibly be trusted with such power?
... I have seen it happen—over and over, with myself urging the moderators on and supporting them whether they were people I liked or not, and the moderators still not doing enough to prevent the slow decay. Being too humble, doubting themselves an order of magnitude more than I would have doubted them. It was a rationalist hangout, and the third besetting sin of rationalists is underconfidence.
This about the Internet: Anyone can walk in. And anyone can walk out. And so an online community must stay fun to stay alive. Waiting until the last resort of absolute, blatent, undeniable egregiousness—waiting as long as a police officer would wait to open fire—indulging your conscience and the virtues you learned in walled fortresses, waiting until you can be certain you are in the right, and fear no questioning looks—is waiting far too late.