From the Model Mayhem FAQ:

How can I remove my forum posts?
If you have posted in the forums, and want to remove a post, you will need to login to your account and click “Edit” on the post to remove the content of that post.

The moderation team typically does not remove forum posts, though we might be able to help hide very old forum posts or posts containing personal information. Please contact the team with links to the specific content and they will let you know if they can help.