Q: How can I unpurchase netfix

Purchases are considered final but you can try the 'report a problem' page to contact iTunes Support and see if they will refund or credit you : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via this page : https://support.apple.com/contact or https://www.apple.com/emea/support/itunes/contact.html

To turn off an app's auto-renewal : View, change, or cancel your subscriptions - Apple Support

If you are reffering to Netflix you must open an account on their official website and then choose a customer plan that best fits your needs and wants.

Hope this helps

The OP wants to do the opposite, she wants to cancel a purchase, not subscribe to it.

(And you don't have to subscribe via their website, you can do it directly in the app.)