Sometimes a discussion heats up anyway. Your first job is to lower the temperature. Slow the pace, lower your own voice, and acknowledge the tension plainly rather than pretending it is not there. A short pause, or a minute of writing, can reset the room.

Then refocus on the question and the ground rules you set. Separate disagreement, which is welcome, from disrespect, which is not. If one student is being targeted or a remark crosses a line, address it calmly and protect the person without shutting down honest inquiry. If a conversation cannot be brought back, it is fine to set it aside and return to it next class when everyone has cooled down.