As for the moment you cannot delete client’s account but you can deactivate it instead.
How to Deactivate?
Login to your account in https://app.storie.co/
Click on your name under the upper right part of Storie
Click Manage Team Members
The one with the ACTIVE button are the ACTIVE ACCOUNTS so to DEACTIVATE you need to click on the ACTIVE button for the account to be deactivated.