While in the forum, click "Forum options" and a second menu will appear below it called "User Control Panel." This is just for the forum.

In the horizontal menu above the main area, click "Friends and Foes."

Now, in the left sidebar menu, choose "Manage Foes."
You'll be taken to a screen where you can add the username of the user whose posts you don't want to see in threads. Example: "Zzz" Copy/paste from another screen or use the "Find member" feature (which is kinda ugly.)
Click "Submit" and that's all there is to it. Posts from those users in the list won't appear, though quotes of their posts will appear in other users' posts.
Use the same screen to remove a user from your shit list by selecting their username in the list, and clicking "Submit."
Accordingly, defining "Friends" in the forum is a pretty worthless feature.
