Welcome to the Apple Community.
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.
Currently, if your purchase happens to be your most recent purchase you will not be redirected to the report form, in this case use the report a problem option from the invoice that was emailed to you.
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.