Set out the 'rules' or expectations ahead of time. Things like, "Smoking will be Ok outside at breaks" and "Please do one song and let the next person have a turn" or "Please do not sing along unless invited to do so by the person singing". That's fair enuf, and it would get rid of lotsa problems before they start. Either that or give offenders a copy of "Everything I need to Know I learned in Kindergarten".