I'm sure there's a Terms of Use policy somewhere around here. I've never had call to read such a thing. My own self imposed Terms of Use - "Be honest" and "Don't be annoying" always seem to cover it :)
You can also get your account closed for spam. Sometimes your account can get closed by mistake. I sent a message to 10 people one time and the system automatically closed my account twice one day by mistake. It can be re-opened only if you send feedback.
How do you get banned?
Are there ways to get unbanned?