grenv: It wouldn't work. As per my example, that user would be able to nag you for a whole week...
A better requirement could be "only replies to posts made after putting the user on block". Which would probably not work very well, if the unsuspecting user would first block someone and then sent the other a message saying "you're blocked now so you won't be bothering me any longer". In this case, he would have inadvertly oppened a hole through the block.

How about if a warning message was issued pointing out that "you have this user on your enemies list. He will not be able to reply to your message" when someone is trying to send a message?