Usage: SHUN [<nick>|<#channel> <nick>] [<reason>]
Allows you to specify nicks that will not be able to talk in channels or send private messages or notices on that specific server. You can also shun a user from speaking in a particular channel; this will not affect private messages at all. To remove the shun, just issue the UNSHUN command with the specific nick you wish to unshun.
To view the shun list type /SHUN. Opers cannot be shunned. However if a shunned user opers, the shun still remains and is effective until it is removed.
/shun Fred be quiet.
/shun #polynomial Fred be quiet.
This can be a rather harsh thing to do to someone because unless you inform them, they will have no way of knowing that they are shunned and their private messages are not being received. It can be used in channels if a particular user is causing problems there, but generally behaving elsewhere.