It is possible to block most accidental or unauthorized purchases made on Google Play, by making sure that the password prompt appears before any purchase is made.

To enable this, first open Google Play and tap the Menu button at the top left (three lines).

1. Select Settings at the bottom of the menu that appears, then under User Controls tap Require Authentication for Purchase. 

2. Select the option For all purchases through Google Play on this device to turn this setting on.