Wow, VoiD, thanks a lot for testing! There's a simple stupid bug, very easy to remove and then it would work as expected. The very simple trouble is that now El_Bitcho is someone else than el_bitcho, I forgot case conversion when using the !ban and !unban command, and it resulted in all the strange behaviour you report. Give me an hour, I'll make sure this is corrected.
As for banning using nicknames (changeable) and usernames (registered). Of course usernames are used for banning, there would be no point is a system banning by nickname.
The !ban command expects username (must be entered all lowercase now, but I will correct this). Only when the driver you want to ban is online, you can use his nickname, it will be converted to username. It is not possible to ban offline people by their nicknames, always usernames must be used (again, all lowercase, correction coming soon).
Same for unbanning, always usernames are used (again, use lowercase, correction coming soon). OK, give me a short while, then pls try all the banning again, you'll see (I hope) the reported strage behavior is gone.