How can I stop downloads from Google Play when net balance is ending (or zero)? In my SIM, the net balance is zero. But, the device shows that it is still downloading applications. Because of this, I am losing my main balance. Also, the battery is getting lower every 3 hours. 

How can I stop this problem?
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.