How to have coupon make monthly subscriptions FREE, not just one time discount?

With a coupon set to 100% discount, I realized it only applies to the first month and after that the monthly payment still gets charged. Is there a way where once a coupon is entered, the discount is permanently free?

This is the bigger picture of what I want accomplished... members can sign up and get a free 30-day trial. 30 days later, if they don't upgrade to the paid monthly subscription their sites will get locked (using Pay to Blog).

However, let's just say they have a way of obtaining a special coupon code to get all the paid features permanently without having to pay...

Is there a easy way to do this?