Currently when someone is banned, the ban will only persist until next server restart. So there is no way to ban someone permanently. The only way admins can deal with that is if they put all of the bans in their config manually. Maybe it doesn't sound so bad, but what if you have to ban a player every few days? There should be a permban command that would save bans to a file (data/banlist.txt or something), which would be read on map load. That would be an easy way to ban players permanently.