At the time of writing, there is no way to delete an account that's been created in your organization via the API. You can still go into the member account and close it using the root credentials, but these don't exist by default. You will need to use the account credential recovery process to log in and close the account.
While you can technically delete your organization via the API, you cannot do this if you have created any member accounts in your organization (since you can't delete them, your organization will never be empty).