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 a four-hour lecture.
Of course, one size does not fit all circumstances, but I found this to be the best tactic for me.
John
