If you are currently on a free trial and want to close and delete your account you can do so from the bottom of the main "Admin" page.
On a Pay-As-You-Go account visit the "Billing Portal" from the main "Admin" page and then click "Cancel" to stop your credit card subscription. You will be billed for any usage up until the day you cancelled.
You Enterprise plan will not automatically renew and will expire at the end of your term. Contact firstname.lastname@example.org if you need to cancel early or adjust the number of checks per month.