Video Games during class
I semi jokingly tell my students at the beginning of the year they can play any game they want on the computers, as long as they write it first. It at least gets a laugh.

For me it takes two things. 

One, I need to be able to see their screens. Given, this is probably much easier with school provided devices. When I moved into the classroom I'm in all of the student seats were facing forward with the monitors facing away from the teacher. First thing I did was push all the desks to the wall so that the monitors are facing the center of the room. I can see almost every monitor from almost any spot in the room. This also helps getting to students to help them since I don't have to squeeze through rows. It does help that I have a pretty big classroom though.

And two, I need to keep them busy. What I've found is that once there's a student playing games it spreads. And the first one usually isn't playing because they're screwing around. They're probably done with their assignments. I throw more assignments at a class than most students will be able to finish - some are extra credit. I also have a list of enrichment activities that they work on when done with labs - Coding Bat, Code Combat, Project Euler, making their own game, that sort of thing. That way there's always something CompSci related to work on. 

We are partially BYOD in that kids are free to bring their devices, but aren't required to. There's always a handful of students that prefer to work from their own notebooks. My rule on that is that they're free to do so, until I catch them screwing around on something else. Then, they have to use the school computers. For most, that's enough incentive to keep on task.