From the Model Mayhem FAQ:

I am subscribed with PayPal - how do I manage or cancel my payments?
PayPal uses their own payment interface and utilize their own Terms of Use which you must agree to and comply with, both of which are managed outside of Model Mayhem.

This means when you subscribe to Model Mayhem with PayPal, your payments are managed directly via PayPal, so you will be best served managing or cancelling your recurring payments directly from your PayPal dashboard.

The following link will assist you with receiving PayPal customer support: