To cancel, go to Subscription in the left sidebar and use the Cancel subscription button.
Deleting the organization does NOT cancel the billing. It's the most common mistake: the organization disappears from your view, the subscription stays active and the charge shows up again the following month.
If it already happened to you
If you deleted the organization without cancelling and kept getting charged, message us and we'll sort it out. Also keep in mind that deleting the organization doesn't release the store connection: if you later try to connect that same store in another organization, it will tell you it's already connected.