Depends.... By Andrea

In general, no matter how tedious or time-consuming it may be, I usually make my displeasure known each time it occurs, first by stopping the lesson, glaring pointedly, & stating that I am talking & he is not, & he needs to remember that in the future. A second offense merits me telling him that I am reminding him of what we spoke of earlier as it seems he was not paying attention to my request to keep quiet when I am talking the first time. A third offense, & I state that this behavior is inappropriate & he has received his last warning; if it happens again, this will be your consequence. Which, depending on grade level, is either going on the "unhappy face" for his teacher to see; removal from the room to a friendly neighboring teacher; & a writing assignment consisting of a letter to his teacher detailing the misbehavior & a plan on how to correct it.

Understandably, this works better in a shorter situation where the regular teacher's reappearance is imminent; I am not sure it would be as effective in a long-term. The consequence would have to be more creative. But I think it is important that you call him on it each & every time he does it & make sure that there is a consequence unpleasant enough to outweigh the peer approval he gets from doing it.