In order to cancel your subscription to any Hextom app, you need to uninstall the app or switch to a free plan (if available). You will not receive any further charges.
To uninstall the Quick Announcement Bar app please visit the Apps section in your Shopify Admin panel. Next to every app on the right you should see a small garbage icon - click the icon to uninstall that app. To see a screenshot, please visit the this page.