grenv: Picture this: someone picks you at random and starts sending you crazy messages. At the first one or two you even reply, but then the other one just get's crazier and crazier and you end up blocking him to avoid receiving such crazy messages.
As long as that user keeps the first couple of replies you sent, (s)he'll be able to go around the blocade and keep sending you messages...

Try to ask someone to send your message to that user instead. And to ask why (s)he has you on their eneemy's list, while you're at it...