For iOS players, if you'd like to disable or enable the possibility of making in-app purchases, or if you want to control the levels of security for each step of the purchasing process, simply follow the steps below:
Head over to your device's Settings
Select 'General' and then 'Restrictions'.
Enter your passcode if you wish to enable the restrictions.
Scroll down to In-App Purchases and choose to completely disable every in-app purchase (turn the settings to OFF). Alternatively, you can manage the frequency of the password request using the Require Password button.
If you want to keep the ability to make in-app purchases, you can set the device to require your password every time you make a purchase.

For Android players, check Edit your payment method - Google Play