You'll see that they posted in threads, but the text of the post will read: 'You are ignoring this user. Show me the post.' <--Not a real link here.
If you click on 'Show me the post' in the thread, it will display their post. It's a handy 'second chance' to not read the post. And as BeMi said, if they try to PM you, they get a message that they can't.