There is a known issue in the Stormpath admin console where deleting an Account from within that Account’s page will sometimes throw an error. If you are seeing this problem, the way to get around it is to delete the Account from the Accounts page.
You can navigate to the general "Accounts" tab in the main navigation header, hover over the Account in question, and click "delete" in the "Action" column in the far right. Alternatively, you can do the same thing by navigating to the "Accounts" page for a specific Directory.
We are currently working on a major overhaul of the Stormpath Admin Console which will address this problem.