Kry explained it well.
We use the forum's infraction system to do day to day banning (insult a member you get an infraction for insulting a member, which I believe is 2 points on the table) and if you reach 5 points you're gone for a sort period of time. Gather 10 points, you're gone for even longer, etc etc.
The only time we perma ban is:
2. The staff come together as a team and decide if it's worth it or not to perma ban the member.
It honestly takes a hell of a lot to deserve a perm ban.
As for posters that can be annoying, as Kry said, use the ignore feature (go to that poster's profile, and you will see "ignore user" button) and you won't be able to see his/her posts anymore.