You can set a PIN to control, limit, disable in-app purchase on Android. It prevents others to buy paid apps, or have in-app purchases, unless password (PIN) is entered. To set a PIN on Google Play and restrict others from buying paid apps do as follows:
1. Open "Google Play" app
2. Tap "Menu Button" and select "Settings" to enter Google Play Settings
3. Scroll down, and tap "Set or Change PIN"
4. Enter your PIN and confirm it
5. Mark "Use PIN for purchases", so the phone or tablet requires PIN before purchasing apps or other content like in-app purchases