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.
EU members, to submit a Deletion of Personal Information request, please go to the
Privacy Contact page and fill out the contact form.

Please note that is not currently possible for non-EU members to immediately remove or delete their accounts from Model Mayhem. Non-EU members can currently remove any information they do not wish to have on the account, and then cancel the account.

In accordance with the new GDPR laws, Model Mayhem will then archive your account after 2 years, and retain what information is on your account for 5 years for legal purposes. After this retention period has elapsed, your account and remaining data will then be deleted from Model Mayhem.

GDPR: Control Over Personal Information Removal
How do I cancel my account?
How do I get my search results removed?