If you don’t want someone messaging you and don’t want to know when they post something, go to your profile page and under Preferences > Users, enter their screen name. They can’t PM you and you won’t get notifications from them.
You WILL however will still be able to see what they wrote. Why you ask? The removing of something that someone posted is almost always negated because in the thread someone quotes what they said. So this feature is useless in effect.