How to permanently disable auto-renewal so you never get surprised by an unexpected renewal charge again.
Go to the service's website (not app, unless it's an app-store subscription). Navigate to Settings → Billing → Subscription or Plan.
Most services have an explicit auto-renewal toggle. Turn it off. You will typically keep access until your current paid period ends, then it won't renew.
After toggling, screenshot the settings page and save any confirmation email. If they charge you after disabling auto-renewal, you have documented proof.
iPhone: Settings → [Your Name] → Subscriptions → [App] → Turn off Renew. Google Play: Manage subscriptions → Cancel. The app itself cannot see these settings.
Note when your current period ends. After that date, verify no charge appears. If you see a charge after disabling auto-renewal, that is an unauthorized charge.