How can I stop downloads from Google Play when net balance is ending (or zero)?
There is no way to disable the Google Play store from downloading apps that you have asked to download, however if you go into the Google Play app settings menu, you can enable the Update on WiFi only option - this will only let updates download when you are connected to WiFi.

You can also completely disable your mobile data connection by going to System Settings -> More -> Mobile Network -> Uncheck the mobile data checkbox.

This will stop your device from connecting to the mobile network data.