In your Profile you will notice a Friends and Foes tab.
When you add someone as a Friend, a request is then sent to the member and he has the option to approve or reject. Once the Friendship is approved, the member will show up under your Friends list and also be added to a dropdown box on the Create Message form. You can also add as many recipients as needed. This speeds up the PM process a little bit.
However, for obvious reasons no approval is needed if you add someone as a Foe. From then on, all posts by your Foes are automatically filtered out. When you pull up a thread you will see a message about the filtered post(s) and can click a link to view the individual post. If a thread has been started by a Foe, you will still be able to view the thread even if you are the only one who has posted a reply.
Friends and Foes alike may added/deleted using the +/- buttons the Friends and Foes section of your Profile. Admins and Mods cannot be added to your Foe list.
I have seen documentation that you must have at least one Friend and one Foe for this feature to work, but haven't tested Friend- or Foe-only stuff yet.