For accounts created through Roku, all billing and account issues must be handled through https://www.roku.com/.
1) Go to My Account, and Sign In
2) Scroll down and click on Manage Your Subscriptions
From here, you can cancel your auto-renewal and manage other aspects of your account.
If you have any problems, please visit http://support.roku.com/home or contact Roku Customer Care at 816-272-8106.