Android is always busy processing tasks in the background which some may use data. Other than being charged for the used data, background data transfer also affects the battery life. To entirely block background data on Galaxy S5, do as follows:
1. Open Settings app
2. While in the tab menu, go to General tab
3. Under System settings, open Power Saving option
4. Tap on Power Saving Mode and then make sure it is enabled
5. Now mark Block background data to restrict applications from using data in the background
6. To avoid any other performance restriction, make sure both "Restrict performance" and "Greyscale mode" is turned off.