From the Model Mayhem FAQ:
How do I unsubscribe, downgrade, or cancel?
To cancel your subscription:
1. Please login to your MM account from a computer with either the Firefox or Chrome browser
2. Then go to My Stuff and then click Wallet from the menu
3. Next, click on the "Cancel Subscription" link underneath the Subscription Info
It's that simple! This action will queue your account for cancellation, and should 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.