I don't think ignore buttons are necessary, but you might as well have it as a feature.
I'd want to have a "blacklist managment" section in my profile, where I could add/remove members from there in addition to buttons somewhere... wait, where would the button be? Not on every post I hope? Just on the particular members profile or something?
Also, I think there should be some indication that your post has been ignored, like how I can "acknowledge" a post without replying to it. You don't want people wondering if their posts has been read or not and not knowing that its just being ignored.