How do I stop in-app purchases?

Yes you can restrict the ability to use in-app purchases. Go to Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > In-app Purchases. Change that setting to Don't Allow.

Many purchases you may see on your bank account are not in-app purchases. What is the exact description of the charge?

Have you reviewed your subscriptions/purchase history by using one of these methods?

• Settings > [name on top] > Media & Purchases > View Account > Purchase History
• Subscriptions and Billing - Official Apple Support
• If you want to cancel a subscription from Apple - Apple Support

The only option I could find is to open Settings ➢ [your app] ➢ Cellular Data = off.

NOTE: If your app doesn’t perform after toggling cellular data (off), then turn it back on or delete the app.

If it was a one time purchase App, you cannot cancel it. You must let it successfully charge you and then you can request a refund for it.

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

If it was an app with a recurring subscription that started and tried to charge you, you must let it again successfully complete and charge you, then cancel the subscription, and then request a refund.

click here ➜ If you want to cancel a subscription from Apple - Apple Support

then here ➜ Request a refund for a cancelled subscription - Apple Support

****, seems I made a mistake when I answered. That's what I get for opening too many posts. My answer has nothing to do with your post. Sorry.