Starting your class room on the first day I have found that after introducing myself that I emediately let my class know that while we are here to learn that no one is allowed to disrupt the education of another student. On that note I encourage students sitting at the back of the class to move toward the front on the excuse that they may hear me better and if they need to ask a question that I may hear them better. Then I stop the class until they comply with that wish. After they have moved forward I then inform them that is thier seat until the lecture portion is over. If after that, they start to be disruptive or talk I stop and ask them what was that? As if they were trying to join the lecture. Then I ask if they want to add to the discussion to please raise thier hand and I'll call on them. This way they know that I will not tolerate interferance while others are trying to learn.