I found out how to disable mobile data for some apps.
Go to Settings →Mobile data (you'll see a list of apps that use Mobile data like Skype, Viber, Whatsapp, and so on) → tap one of them (e.g. Viber) → scroll down to Disable mobile data or similar → Tap it and agree to what is mentioned there.
Now you disabled Mobile data for Viber. You might be having lots of apps but the effort pays off! You may limit mobile data usage to 0 since it works too. :)
