I'm really glad I don't have to deal with what you deal with AB.
What would happen if you said during the preflight briefing ( with the entire crew) " Righto, you know the flight time and the expected weather, I don't want any of us to use our mobile phones onboard while the engines are running today, unless you are in the bunk, as I think it is starting to become a distraction. Does anyone have anything to add? Ok let's go to the aircraft and have a good day"
That way, everyone knows what behaviour is expected of them and you haven't singled out your F/O, also, if he or she now goes ahead and does it anyway it's a sure thing that you don't really have command of the ship and you can move quickly to address that.
What do you think AB?