If you want to disable the in-app purchases on your device, follow these steps:
  1. Go to your device's Settings > General > Restrictions > (Enable Restrictions)
  2. Tap on In-app Purchases
In-app purchases are now disabled.

If you want your device to request a password for every purchase you make, follow these steps:
  1. General > Restrictions > Password Settings
  2. Select "Always Require"
For more information on disabling in-app purchases, please visit Apple's website. (this link will redirect you outside of the game)