Yes, most accounts are automatically setup to renew automatically.
Bitcoin, CashU, iTunes, Gift Card, Ripple, or any gift card payment methods (Mint, ecoPayz, Neosurf, Ticket Surf etc) do not automatically renew.
This is outlined in the Fees section of our Terms of Service, which you agree to upon signing up.
You can cancel the account at any time to prevent the auto-renewal. Your account will remain active for the remainder of the time that you have already paid for, then cancel at the end of the paid time. Once cancelled, and account cannot be reactivated, and you will have to sign up for a new account in order to use the service.