You can use Restrictions to prevent unintentional or unauthorized In-App purchases.
To do so, perform the following:
- Tap the 'Settings' icon.
- Tap 'General' > 'Restrictions' > 'Enable Restrictions'.
- Create a Restrictions passcode, then re-enter your passcode to confirm.
- To prevent in-app purchases, tap the switch to turn off In-App Purchases.