You can cancel your account at any time via the Billing page.
Scroll down to the bottom of the page, then click on the red "Cancel subscription" button.
You'll then see a popup which allows you to leave some optional feedback.
If you click "Proceed with cancellation" you will move onto a read-only Free plan, and you won't billed again. You'll continue to have access to premium features until the end of your current billing period.
We won't delete your storyboards. We'll keep all of your data intact unless you tell us to remove it from our servers (which we are happy to do).