Yes. You can close/downgrade your account at any time by visiting your Account Settings (top right corner) > Plan > Close/downgrade account. You'll then receive further instructions via email.

The account will be closed/downgraded to free at the end of the account's billing cycle. Since we post charge, you can expect one final invoice on the last day of the account's billing cycle. After this payment has been made, the account will be closed/downgraded immediately.