I don't think there is a way in game to block another players messages. If the messages are rude or degrading you can report that player with the little red button under their picture in the message. If its just annoying you might be able to contact support and explain the situation and they might be able to do something. I am not sure if they can block him from sending you messages or not, but you could try. If your not sure how to get to support, right click in game will bring up a pop up a box, click the support link, then put in a ticket and they will send you an email. Hope this helps.