The Silence Command may be used to effectively prevent private messages from another user, commonly known as "PM", from reaching you.
If you desire to invoke the silence command, type the following in the input line of the Mibbit client:
/silence +nickname (or /silence +host - if (s)he keeps changing the nick)
Replace "nickname" above with the nickname you wish to silence.
To remove silence protection you have set, type the following:
The /silence command blocks both PM messages and notice messages, which are less frequently used.
The result of silencing another user 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 the Mibbit client, any active nicknames that are silenced will become unsilenced upon restarting the Mibbit Client.