If the person is using mulitiple steams get his ip via HLSW and do the same thing of banid 0 "22.02.222" or whatever his ip is
go in your server and type in console amx_rcon exec server.cfg or just do a map change
/end
This person will be banned indefinately. Even if one of your admins uses amx_unban on this person it will reban him on map change so only those with FTP access can affect your bans.