Great question! If the app charged you through the Shopify billing system, meaning that the charge appeared alongside your subscription charge, then removing the app from your store should stop all future billing. That said, apps have their own billing cycles so there is a chance you'll see one more charge if you removed the app within its current billing cycle. If the app charges you outside of Shopify, then you'd need to contact the app developers so they can remove your billing information.
Let me know if you have any questions!