Because memberships provide access to unlimited downloads, an annual commitment is required on all memberships. Memberships can be purchased as one lump sum payment, or in 12 monthly installments over the course of one year. You can turn off the annual auto renew of your account in your Account Settings under the Membership tab.
Monthly subscriptions can turn off the renewal of the membership once the 12 monthly installments have been completed. An annual membership can be canceled to not renew again for another year, but the membership will remain active until your end date.