Go to Settings --> Privacy --> Personal data --> Manage ad preferences.
There are so many checkboxs there (all are on by default), and there is a _very very long_list of trusted partners (and use of data). It took me more than 30 minutes to uncheck all of them.
There isn't any "unselect" all.
I have to use Viber for some communication purpose. And this is so painful. I'm writing here just in case... Please note I'm using some Europian number for my Viber.