You can cancel your Back In Stock (In Stock Alert) Apps subscription any time by uninstalling Back In Stock (In Stock Alert) App from your store. Your subscription will be canceled and no further payments will be taken.
Here are the steps to uninstall the Back In Stock (In Stock Alert) App from your Shopify Store:
Go to the Shopify Admin Panel.
Go to Apps and Sales channel settings.
Click on delete option
4. Let us know why you want to uninstall and remove the App.
5. If you'd like to cancel the subscription but continue using Back In Stock (In Stock Alert) App for the remainder of your billing period please contact support.
Hope this helps :)