To block Private Messages (PMs) from a specific user, follow these steps:

1. Click "Your messenger" in the upper right of any message board page.
2. Click "Address Book" (the third tab).
3. Partway down the page, you should see an area that says "Add a member".
4. In the "Member's name" field, type the member's username exactly (it is case-sensitive),
5. Set the "Allow this member to message you" menu to "No"
6. Click the "Add this Member to my address book" button.

If the member with that username tries to send you a PM, they will see this:

This member has chosen not to be contactable by the Ikonboard Messenger.

You will not receive messages from that username.