The Silence Command may be used to effectively prevent private messages, commonly known as "PM", notices, and CTCP events from another user from reaching you. For channel messages you have to use the /ignore command (client side).
If you desire to invoke the silence command, type the following in the input line of your IRC client:
Replace "nickname" above with the nickname you wish to silence.
If they keep changing the nick use:
(wildcards work too, e.g., /silence +mib_* or *@*BEEFB9E5.IP).
To remove silence protection you have set, type:
To show the current silence list type:
The result of silencing users is transparent. You won't know they are trying to PM you and they won't know you aren't receiving their PMs.
After logging out of your IRC client, any active nicknames that are silenced will become unsilenced upon restarting your IRC client.