All answers below was basically the same question, and user who posted was calling us lazy, and worse. Yet he wasn't prevented from posting for a quite long time - he only needed to switch usernames.
I think he exploited some kind of bug in the auto-ban - usually banned users are recognized not only per username, but also per IP and other means. Somehow, he wasn't.
Any chance this will be prevented in future? Or is it something we have to live with?