At the time of writing, there is no way to delete an account created in your organization via the API. We can only imagine that being able to programmatically delete a member account created in an organization will be a highly requested feature, and will be addressed soon. You can still go into the member account and close it using the root credentials, but these don't exist by default.
While you can technically delete your organization via the API, you cannot do it if you have created any member accounts in your organization (because you can't delete them, your organization will never be empty). This should improve in the near future, but is still worth being aware of now.
