This will add a permanent kline on your system. A kline will prohibit users from joining the server. The security parameter uses a mask of user@host and a reason. These will stay even if your server is shut down and can only be added through the AdminServ command set. Other klines added with the /kline command are temporary and will not remain after a /rehash or you stop your server.
/as security kline add user@*.test.org Flooding the server
If you run an open chat server, then you should make sure to go through your klines and remove old ones. Since these klines will never go away on their own, you may end up with people being banned for a very long time. Even if a ban only affects one user, many users will reform and begin behaving eventually and most bans will block chatters who use the same ISP.