Please use the following instructions to do so:
  1. Navigate to Settings -> General -> Restrictions.
  2. Select Enable Restrictions.
  3. Enter the Restrictions Passcode and then enter it again to confirm. Make sure to not use the same password as the one you use to unlock your device.
Once you select Enable Restrictions, you will see a list of actions and applications allowed on your device. To disable in-app purchases only, set the In-App Purchases option to off. To completely turn off the ability to make purchases, set the following options to off: iTunes Store, iBooks Store, Installing Apps, and In-App Purchases.