Is there any way to completely block any app from using data (Android 6.0)
One simple way to manage what application will use your mobile data is using Triangle by Google. Get it on APKMirror or APKPure and install it with ADB. Plenty of alternatives are offered here.

There are also a great number of ways you can save on mobile data usage. You can use less data with Chrome's Data Saver. You can set mobile data usage warning and limit. For example, set the limit at 1 Gb and the warning at 0.618 Gb. You can use Firefox Focus or Firefox with uBlock Origin. If you are rooted, you can get AdAway on F-Droid.