I believe confrontation (privately) would probably be the necessary option if you aren't lecturing. However, I make a point to walk towards the disruptive student(s) while lecturing, standing next to them while addressing the class. The conversation always ends because they won't raise their voices above my classroom-projecting voice. Add the fact that they are now theoretically 'in front' of the class with their classmates facing their direction to look at me.  I continue for a minute or two then return to the front of the room. 

Repeat as necessary. The most I needed to do this was twice in… >>>