This is a snippit from an a FAQ on the Undernet IRC Network. The full text can be found here.

1-32) Help! This extremely obnoxious person keeps harassing me with messages/flooding me. What should I do?

One of the first commands that a newcomer to IRC must learn is the magic /ignore command. With this command you can ignore people flooding you or your channel, or harassing you, or whatever. The syntax of the ignore command is:

     /ignore user@host ALL

To find the user@host for a person, do a /whois nickname, or a /who nickname. If you just wish to ignore messages from the person you may do a /ignore nick MSG. /help ignore will give you more information on this command. You can use wildcards (* and ?)in the user@host. Thus to ignore everyone from a *.com site, /ignore *@*.com all

On the Undernet, you can also use the "/quote silence" command to counter people flooding you. This cuts flooding at the *local* server unlike /ignore where your client continues to receive messages even though you may not see them, and causes your client to ping timeout in many cases. The syntax is:

     /quote silence +user@host
     or /quote silence nick