How to deal with uninterested students during an optional exercise session?
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.