From the Model Mayhem FAQ:
How do I remove my profile from the site?
Well, why the heck would you want to do that?! But if you insist, go to My Stuff > Settings > Cancel Your Account.
Please be aware that Model Mayhem doesn't completely delete profiles, though cancellation renders profiles invisible to the public and search engines. Note that after account cancellation, your profile and images may still show up in search engine results. This is not within our control.
To speed up the disappearance of your profile and images from search engine results, take these steps before cancelling your account. Delete your images in My Stuff > Manage Photos, remove all personally identifying info (such as email addresses and variations of your name) in My Stuff > Edit Profile, and change your display name to something generic in My Stuff > Settings > Change Display Name. You may also want to delete tags from your profile and tags you've left on other people's profiles, especially those that mention your name or personal information.
If you don't want to show up in Google searches, also see here.
If you've set a custom URL that you don't want to be associated with anymore, see here.
If you delete your images, occasionally image variations (avatars, thumbnails) will hang around and keep showing up in search engine results. If this happens, contact the mods with the image URL(s), and the mods will manually delete the image(s) for you.