Simply delete the Simple Affiliate app from your Shopify Store.
If you forgot to delete the app before your next bill payment, no problem! You can request a refund by contacting Shopify Support: https://help.shopify.com/en/questions#/contact. All you have to do is let them know the name of the 3rd party app in question. Shopify support will then reach out to us at Simple Affiliate, to request a refund on your behalf, which we will approve. It's important to remember that Shopify is responsible for refunding merchants, so you must contact their support team to initiate the process.