This is the easiest way to block or disable background data when using mobile data for any specific app. You can also look at the data usage graph for both mobile and wifi data. I know that I can also get to this screen by going through the Data Usage settings but if the app is consuming little data and if I insist of turning off the background data, then I need to scroll through the long list of apps.
Step 1 : Go to the "App info" screen.
Step 2 : Go to the "Data usage". You can find the checkbox to enable the block.