From what I understand, the Ignore User function will block POSTS (not PMs) from the users on your list.

But, if the user is also the OP of a new thread, I'm guessing the software wants you to still be able to see the thread (because it might be full of posts from non-ignored posters you like), so original threads by ignored users are still visible to you. Likely this is the desired behavior and you may just have to "manually ignore" (haha) them.

Paul