One of my groups meets in a small room and it is noticeable when a separate conversation is going on in the room. We have several members who can't hear well and so they chat a little too loud to their neighbor, thinking they are quiet. The best way I have seen to control it is to stop the meeting till they notice and are quiet. It's up to the speaker not to try to talk over them. They get the gentle message and the extra chatting usually stops.