Usage: MKILL <IP address|hostmask|#channel> <reason>
This command will mass kill users from the same IP address, hostmask or channel. It is normally used when a large quantity of users from the same address or channel need to be removed from the server.
/mkill cloning
/mkill #bots botnet channel
This is especially useful if the users are changing their nicks because it allows you to kill by address. It is a good idea to check exactly who will be killed by this command before you send it by using the WHO command.