Ignore user is a sticky wicket. It really is a mess if you would allow a user to restrict replies to non-ignored users.
And then, what about quoted text? How could you prevent the ignored user's post from being quoted by an un-ignored user, so in effect, the ignored user's text is seen?
I just got a suggestion/request from one of my users who uses the "Ignore User" feature a lot. He suggested an "Ignore Thread" feature. That would be easier to implement.