Certain methods of reading will still allow one to encounter the posts. For example, browsing by recent posts is just full-text posts by everyone, regardless of what's on your list. I'm also told that one will see that a post has occurred from ignored users, but the post will have text like "(You have this user ignored) ".
You can block users from sending you personal messages through your profile.
This is a public forum so you cannot block anyone from seeing your post and responding, despite requests from people to have such a feature.