Welcome to the Apple Community.
You can set up your device to require a password for purchase from the app store either immediately or after 15 minutes of being previously used. (Settings > General > Restrictions > Allowed Content > Require Password)
More often than not this stops unwanted purchases by others, but does not stop In-App purchases, to disable In-App purchases, go to Settings > General > Restrictions > Allowed Content > In-App Purchases > Off.
Apple's policy on sales of digital content is that all sales are final. If however you wish to appeal to Apple, select the content which is causing a problem and use the 'Report a problem' button in Your Purchase History on your computer.