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From the Model Mayhem FAQ:

How do I cancel my VIP or Premium membership?
Wha? We didn't hear that. We're sorry, you said what? If you feel you really must leave, simply log into your account, head to the "VIP Wallet" or "Premium Wallet" link in your My Stuff area and click the "Cancel" button. Your subscription will be canceled at the end of your current cycle. There are no refunds however, so if you've already paid for time, you'll remain a VIP/Premium until that time expires. If you have any other questions or concerns, please send in a request via our Contact-A-Mod system and reference VIP as a question topic.