From personal experience, you can be banned for 12-48 hours even if you're totally clean. I think it has something to do with switching accounts or characters too many times in some time period. But it seems pretty inconsistent to me. Also, bans sometimes don't register until you log off and later try to log back in. Once I played a lot on the day before I went on a weeklong vacation during which I did not play at all, and when I came back I was banned for two days from the first time I tried to sign on.