Here are some steps you can take to disable in-app purchases on your mobile device:

1. Open the Settings menu on your mobile device.
2. Scroll down to find the 'General Settings' and tap on it.
3. Choose the 'Restrictions' settings and enable it.
4. Enter a 'Restrictions' password if prompted.
5. Scroll down to the 'In-App Purchases' option and set it to 'Off'.