A similar thing happened in a recent pub session (in a pub that is known as a "folk club". 2 very loud young ladies were screaming with laughter at something on their phones while our singer was barely heard. He stopped singing, turned to them and said, "I'll start again when you two have finished". It worked a treat! No fights or hard feelings.
The same singer - in another session- also told the person next to him to stop flicking loudly through his massive song folder "I'll carry on when you stop that".