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.