People are banned for being assholes, typically repeatedly.
There are several behaviors that will get you an automatic timeout. Example: posting nudity (no nips, no bush, no franks, no beans). These sorts of infractions result in 1 to 2 week timeouts, i.e. temporary bans. If you were to cross this line repeatedly in a fashion that made it clear you were just being an annoying dick, you could get a permanent ban.
Racism is the other big no-no around here. If you start calling someone out for being a jew, expect a ban. Depending on the severity of the infraction, you might get only a timeout, but fuck that up twice, and you should probably start focus grouping new handles.
In other cases, the bans are more subjective. There was a gentleman some time ago who had strong opinions on clothing, so strong that he quite vilified several other members and generally became a scourge. He could not play nicely in the sandbox, and finally was thrown out. Another gentleman, who now posts under a different name, trolled many people, including mods, and eventually they got tired of his act. His new incarnation is more respectful.
Spammers are also banned, but in most cases they are instantly banned and their posts are deleted, so you won't see their legacy posts hanging around.
The general rule is that SF is like a bar. We take all comers, but at a certain point, if you're enough of a dick, you're going to get thrown out. Act like a dick to the bartenders (moderators), and you may find yourself on your ass in the street on an even quicker trigger.
Most will note that moderation here is pretty fair. If anything, the complaint is that moderators are too slow to pull the trigger.
So, with kindness, please don't be a jerk.