"Time-out."  I use the referee hand signal to make my point.  "Lil' Ray, hold that thought a minute."  Then I address the rest of them.  "What are the classroom rules?"
A half dozen kids roll their eyes and groan.
"Do we gotta say it?" somebody pipes up.
"Until we start remembering to follow it, yes," I insist.  "Or we can go back to the classroom and diagram sentences.  I'm good either way."  I make the motion of a choir conductor's baton.  "All together now.  What's Article Number Three of our Classroom Constitution?"
An unenthusiastic chorus responds, " We encourage vigorous debate.  Civil debate is a healthy democratic process.  If one cannot make one's point without yelling, name-calling, or insulting others, one should develop a stronger argument before speaking further."
-The Book of Lost Friends, Lisa Wingate