SILENCE

Usage: SILENCE [-|+]<nick|nick!user@host>
The silence command allows you to ignore a specific user. For example, if a user is flooding or harassing you, the user can be silenced. When you silence a user, all messages, notices or ctcp events initiated by that user will no longer reach you. Others see this user's messages normally.
You can specify a nickname or a mask, any user that matches that mask will not be able to send private messages or notices to you. To remove a silence, issue the same command with a minus sign "-" in front of the hostmask or nick
If you specify a nickname, then it will add the *!user@host mask that matches the nickname you specified, if you only want to silence a particular nick, you need to do /SILENCE nickname!*@*
Examples:
/silence
/silence rowdy
/silence -rowdy
/silence Flooder!trill@pm02.digex.net
This is a good command to stop people from annoying you. You will see anything they say in channels you have in common, so you won't miss anything important. Yet you won't see any private messages. Silencing people is a very good tactic that is not used as often as it should be.
There may be many immature people who get on your nerves, simply not dealing with them is best. Then you can enjoy chatting in peace with the people who do interest you.
Note: many clients provide a client-side command with similar functionality, it's usually called /IGNORE. The difference is, that SILENCE will prevent the messages to even be sent to you, while with /IGNORE, the client still receives them but doesn't display them.