How to deal with uninterested students during an optional exercise session? Last week I taught my first exercise session. I am a PhD student so the kind of classes that I will teach are exercise sessions about what the Professor explained in the theory class. They essentially have to work problems and I have to explain and help them with their approach in solving them.

Students of the same year are divided into small groups. I had about 25 students and from what I noticed: 17 were working hard, 7 were working more slowly with less interest and 1 was watching videos with his smartphone.

I tried to get his attention in a kind way several times (by going to him and telling something like "I know that videos are funnier than this theoretical stuff but if you want to practice for the exam you should do also exercises") but even if he stopped and started to read the exercises then he always restarted watching videos.

Since I will not assign grades and this kind of class is not mandatory, his participation will change nothing in his final grade.
So at a certain point I just wanted to ask him why is he attending.  There are many seats outside the class where he could sit and watch videos, but I just stopped to get his attention and I left him watching videos. I made this decision because he was watching videos/surfing the web without audio and without annoying the other students.

What is the best way to deal with these situations?
Do you prefer more or less authoritarian style? 

More authoritarian: "If you come to my class, I assume you wish to be taught by me and respect my time. In other words, you are to work on your assignments. If you do wish to watch videos or to Facebook, that is fine, but please do not do that in my class - it is disrespectful to me and in fact also to your fellow students who come here to learn. Feel free to leave to watch, and to come back when you are done and wish to return to the exercises."

More liberal: "It is everyone's own responsibility to learn and I will help you to understand the material and how to master it. If you do not wish to engage in it, that is your free decision, but you need to be aware that this is your own responsibility to engage with the material in order to benefit from it [and pass the exam]." - say it once at the beginning, and then perhaps one more time in the middle of the semester. That's it.

Modify as required, this is just a coarse line of action, YMMV.