Just email us at firstname.lastname@example.org and we'll immediately process your request. Cancelled subscriptions remain valid until the billing period ends (i.e. if you're paid up through the 20th of the month, your cancelled account will remain fully functional up to that point).
Legacy subscriptions prior to 9/24/2013 are managed externally via Amazon Payments. You can login and manage your Amazon Payments account at https://payments.amazon.com using your existing Amazon login. From there you can cancel the subscription at any time.