We've had a few people banned here, for illegitimate reasons. We need a guide for admins to know when to ban.
Vandalized three times in a row, with prior knowledge he was vandalizing the articles.
Personally attacked an user for no good reason multiple times.
Made bad articles (ie. Racist, sexist or other wise vulgar comments) Multiple times.
When definately not okay
You don't like the user.
Cuz' you can
For no good reasons.
Keep in mind that there can be inbetween times on when to ban people, use good judgement and assume good faith.