For individual accounts, you can cancel your subscription by clicking the avatar in the top right of the toolbar when signed in to Book Creator. Click on Subscription.
Then click on the Cancel subscription link in the popup. You'll get a message about when your cancellation will be effective (that is, at the end of the billing period you've already paid for). Click Confirm Cancellation and we'll cancel your subscription.
If you change your mind before the end of the billing period, you can simply follow the steps above and then click on the Reactivate subscription link to carry on as before.