Be logged in on the site, then use the deletion link (which can be found under My Stuff > Settings, too).
URLS:
http://www.modelmayhem.com/
http://www.modelmayhem.com/mystuff/settings/cancel_account
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.