I won't pretend to say that I have inside knowledge of what goes on "behind the scenes" here, as I don't.
With that said, I am a moderator on one of the largest watch forums on the web. I'm pretty intimately familiar with our ban procedure there, and from discussions I've had, our procedure is pretty standard across larger/well visited forums.
The first thing is that there's often a lot going on behind the scenes which members can't see. If a member makes a mess, a moderator usually cleans it up and doesn't leave a record that's generally visible(although on the forum where I moderate, we use Vbulletin's "soft delete" option where moderators can still see deleted posts). Also, following an incident that requires extensive clean-up, the member(s) in question will usually get a PM from a moderator explaining the situation, reminding the member of the rules, and warning them of further consequences. With many folks intent on causing trouble, this alone can often escalate things out of control.
Second, we escalate a through a series of "time outs". Our standard is 3 days for the first infraction(and usually only after a series of warnings), followed by 7 days, 30 days, and then perma-ban. Perma-Ban requires the consensus of several moderators, and must be approved by the site owner.
I'll also add that often times a perma-ban is invoked after a seemingly minor infraction. When considering a perma-ban candidate, we consider their entire site history, and even if the infraction in front of us is minor, it can be "the straw that broke the camel's back."
Finally, our TOS(which are fairly standard) give us pretty broad scope in banning members. Members with a consistent pattern of relatively minor infractions that stray just over the line often get perma-banned simply because they consume too much moderator time. This may sound harsh, but with a big forum and relatively few moderators, the community is better off getting rid of a member that contributes very little but constantly makes a lot of work for the moderators.
