From the Model Mayhem FAQ:
How do I unsubscribe, downgrade, or cancel?
To cancel your subscription, please login to your MM account from a computer with either the Firefox or Chrome browser, then go to your VIP wallet and click on the "Cancel" button underneath the Cancel Subscription heading. That will ensure that you're not billed again. Any subscription time you have paid for will remain on your account until your subscription ends.
If you have any other questions or concerns, or are having trouble cancelling your subscription, please contact us.