If you don't want to receive PMs from a specific member, you can have them deleted before they reach your inbox.
Go to User Control Panel
Click Private Messages
Click Rules, folders & settings
Under the Add New Rule section, select:
Sender - Click Next
is user - Click Next
type in their username - Click Next
Delete Message, Click Add rule.
This will automatically delete all new private messages sent from that user.
You can also report any private message to the moderator by clicking the report button at the top of the message.