From the Model Mayhem FAQ:

How do I remove friends?
Removing friends is just as simple as adding them!

First, go to Friends in My Stuff and find the friend you want to remove.
  1. Next to their information on the right there will be a check box. Check it!
  2. At the top of the page, there will be an X button - press it to remove that friend.
Here's a screenshot of what that should look like: