I've added a feature that will allow you to block PMs from individual users.
--> Edit Profile (top left corner of this page) --> User Profile --> find the text are that says: "Block Private Messages from these Users" --> Enter the usernames you wish to block one per line.
Be sure "Allow Private Messages" is set to Yes or all PMs will be blocked. When someone on your blocked list tries to send you a PM they will be notified that you are not accepting messages from them when they hit the "Send" button on their PM. Hope this helps.