Here is how to block a sender's email address or domain:
Sign in to your control panel
Click on the Settings tab
Click on the Blocked Senders sub-menu item
You'll be taken to the Blocked tab on the Approved and Blocked Senders page. This tab lists email addresses and domains that are blocked from sending mail to your domain.
Click on the Add New button
You'll be presented with a dialog box with a large text entry field where you can list the email addresses and domains that you want to blacklist.
Changes to your blocked sender list will normally take effect within 5 minutes.