Actually there currently isn't a way to ban people while they are not on the server, because mani admin plugin doesnt allow you to add a ban to the server unless they are on the server, now thats not to say that zero could possibly customize mani admin plugin a little bit and add a command that can add a ban while a person isn't on the server and the only other way to add a ban to the server w/o them being on the server is through the server files by just adding the steamid, ip, etc....
Also, just to commant on Ownzs' suggestion that would be agood idea as Rage said and they way things are right now is the best because there is really no other way that would be better because even if we were to change the way bans are added than that still means there will always be a process to get someone either banned or unbanned from a server and could take an even longer time to get unbanned if there are only a few people that can issue an unban, no offence Ownz