This one is sounding a bit like the good old bodhran conversations. I don't know the situation in this example but as a general principle, I am against the idea of telling somebody they can't join in. Assuming we are talking about reasonable people, as this case seems to be, maybe a please keep it down a bit or possibly some construcive advice is likely to provide a better solution.

In about 20 years of playing in folk clubs and sessions, I have yet to suggest that somebody should not be joining in (although some situations might be intolerable - I've not come across a really bad situation like say a badly played trombone in an Irish session) and the closest I came to asking somebody to stop occured fairly recently. This was in an event that I am paid in beer money to keep going (it's a sort of cross between a jam session and ub entertainment) and a local bodhran player turned up with something that I think was called an ocean drum (looks like a double sided bodhran with ballbearings in side it). I simply could not keep time which the delayed sound that the bearings produced and did tell him that if he wanted to play it, he would have to sit as far away from me as possible.

Jon