You can download and play Guns of Boom for free, but there is an in-game currency, that is used for speeding up the game process. The in-game currency can be purchased with real money via App Store, Google Play and Amazon.

To disable the in-game purchases on your device, follow the link next to the store you use:
for Google;
for Apple.

If you want to keep the purchases enabled, set up the device so that it requires a password whenever you try to make a purchase (select “Always require“). You can also set a window of 15 minutes since your last purchase after which you will have to re-enter the password.