If this involves a subscription then please read this link to learn "How to cancel a subscription from Apple" Click here --> If you want to cancel a subscription from Apple - Apple Support
Note:
• "If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew." The "Expires" date means the current subscription will just run until then.
• Deleting an app does not cancel a subscription.
• If you don’t cancel trials more than 24 hours before your renewal date, your subscription might renew.
• If you don't have an Apple device, see the options near the very bottom of the support document
"If the subscription that you want to cancel is not listed", click here --> If you want to cancel a subscription from Apple - Apple Support
If you still can't find a subscription, contact Apple Support using the link at the end of the support article.
FOR REFUNDS for either a purchase or a subscription see how to "Request a refund for apps or content that you bought from Apple" Click here--> Request a refund for apps or content that you bought from Apple - Apple Support
If a reporting a problem request is unsuccessful you can also try clicking here --> Choose your country or region - Official Apple Support Select your country, then a product.
For future mitigation:
If this happened on a device you control you may wish to require a password for App Store and iTunes purchases --> Require a password for purchases in the App Store and other Apple services - Apple Support or 'Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support' --> Use parental controls to manage your child's iPhone or iPad - Apple Support "to block or limit ... features on your iPhone, iPad, or iPod touch"
"You can use Screen Time to prevent unintentional or unauthorized purchases from the App Store and other Apple services. You can require a password for purchases, prevent certain types of purchases, or disable purchasing completely." - Use Screen Time to turn off in-app purchases on your iPhone or iPad - Apple Support See how to 'Use Screen Time on an iPhone, iPad, or iPod touch' by clicking here --> Use Screen Time on your iPhone and iPad - Apple Support
If this happened on a child's device you can approve what kids buy with Ask to Buy - Approve what kids buy with Ask to Buy - Apple Support
Realize Apple's terms of purchase for most countries are effectively, "All Transactions are final" (check terms at: Legal - Apple Media Services - Apple then select your country). Apple does refund on a case by case basis, but they can also refuse.