This is the command you use in the Chatterbox to block all messages from another monk. The full form is
/ignore SenderNameYou can enter such a command in any text box where, if you typed any other text, it would go to the public Chatterbox.
Note that unlike /msg, /ignore parses on the first whitespace only, so you never need to put brackets around the monk name (although you may if you prefer). Example:
/ignore Mr. MuskratTo see a list of the users you are currently ignoring, go to ignored users.
The symmetric command to /ignore is /unignore. This removes the named monk from your ignore list.
As a convenient alternative, you can go to ignored users; next to each listed name will be a link which, by merely clicking on it, removes that person from your ignore list.
Further info is available in Using the Chatterbox: Private Messaging.