From the helpfile:
/ban [-kruN] [#channel] <nickname|address> [type]
Bans someone from the current channel using their address. To do this, it first does a /userhost on the user, which gives it the user's address, and then it does a /mode # +b <user address>.
Note: This command uses the IAL maintained by mIRC.
If the address isn't in the IAL it does a /userhost.