Did you try these instructions
http://www.amazon.com/gp/help/customer/d...
Somewhere they state if you have never benefited from the prime account trial, you may be able to get a refund of the automatic billing regular account fee.
On the left hand side (after clicking "manage your Prime Membership") you will see a link "END MEMBERSHIP" directly under the date you subscription renews.
you have contact Amazon and cancel it
It's there.
