A zline will reject any attempt from a specific or masked IP to contact the server. The zline acts as a sort of firewall.
ADD - Add a permanent zline mask.
AUTO - Set automated zline options.
DEL - Delete a permanent zline mask.
LIST - List all the permanent Zlines on your server.
NUMBER - Sets the number of seconds that zlines will remain active on the server.
/as help security zline <command>
Zlines are the strongest form of server ban. As with any ban, make sure to set them no broader than necessary and to periodically review the list and clean out old entries.