Mmhh, this is strange. Using the Chatterbox: Private Messaging states clearly that /ignore should also stop private messages:

To ignore a user, just type /ignore followed by a space and the user's name. This option will not only refuse private messages, but also you will not see any public messages that they post.

So probably the documentation has to be fixed - or, even better, the feature so that it works as promised.

-- Hofmator