how do i stop a payment i didnt subscribe to?

Itunes

If you received an email about an unfamiliar purchase, it might be a phishing scam. Don't call, email or click on any links. Just delete the email and you'll be fine.

To confirm if the purchase was really made, log into your Apple ID/iTunes account directly and check your billing or purchase history. If there really is an unauthorized purchase shown there, you can also file for a refund from there using the Report a Problem form.