Similar to background data restriction you can also restrict background data for WiFi. Follow these steps to restrict background WiFi data;

1. Open settings and goto data usage:

   

2. Click more placed on top right side:

   

3. Click on show WiFi usage:

   

4. WiFi tab will be shown in right side as shown in figure select that:

   

5. Then again click more and select restrict networks:

   

6. Select your wifi for which you don’t want to use background data:

   

7. Now it is done.