Refund on unwanted purchase

All you can do is try.

Request a refund for apps or content that you bought from Apple - Apple Support

In most cases all transactions in the App Store are final, but at times Apple may provide a compassionate refund. It should never be expected though. Any purchase requires double clicking the side button with the amount showing on the screen, followed by authorization with FaceID/Passcode. It cannot be done by accident and if you have given someone else your Passcode, you have authorized them to use your device and that includes for purchases.

Depending on how your purchase was authorized, there are many ways to prevent it, but would need more details for which method to use.