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.
- Next to their information on the right there will be a check box. Check it!
- 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: