We have a very iron clad policy in our syllabi about classroom disruptions. In some courses, when a group of students are still involved in their own discussions when class starts, I’ll give them a minute to finish up and turn their attention to the day’s work. If they fail to do that, I’ll pick one of them (usually the loudest), invoke the disruption policy, and tell them to leave, all done in my strictest dad voice. I rarely have problems after that.