If you want your device to request a password for every purchase you make, follow these steps:
  1. Open the Google Play Store app
  2. Tap Menu > Settings
  3. Select Require authentication for purchases
  4. Choose For all purchases through Google Play on this device
  5. Follow the on-screen instructions
For more information on disabling in-app purchases, please visit the Google Play website. (this link will redirect you outside of the game)