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From the Model Mayhem FAQ:

How do I close, cancel, or delete my account?
To close your account, simply go to My Stuff, then Settings, and click the Cancel Account button at the bottom. We will be sorry to see you go!
Important Message from Model Mayhem:

Before you cancel your account, if you do not want any images or references to your old account to be available in search results we recommend removing your images, updating your name, as well as deleting any old comments and TAGs.
As a note, we cannot delete your account, but we can help you close your account if you need help. Just contact us.