Usually the problem is, that you don't want to ban the user, but the behavior. And - reddit being mostly a textual thing - it is represented by words. Banning the user is unfair, plus they can just create a new account anyway.

So you try to be alert for people who use certain phrases. Racism, xenophobia, conspiracy theories, paranoia, extreme pettiness, severely abrasive attitude, choosing beggars, and so on.

And flame wars, and people jumping on others. And of course if there is a group of people showing such unwanted behavior it takes no time for the powder keg to go off.

I think having sort of simple but a big ambiguous rules are okay if there are very broad but exact rules too. (Eg. Rule1: don't be a dick, Rule2: no harrassment, no xenophobia, no posting of personal information, etc.)

Moderators and the community should be proactive. (I find that the best tool is reports, because that signals what people find problematic. Sometimes they are just annoyed that some newcomer posted that again. That's okay, but most of the time users report spammers, crazy serious racists with too much time, and the occasional lost redditors' posts.)