Snail, of course you have authority over the people who come through the door of your club. The club is running the evening, and is perfectly entitled to decide who should and should not be invited to sing.

Perhaps you choose to avoid that responsibility, and simply provide a venue and tell the audience to get on with it. Perhaps you are fortunate enough to have a strong enough body of singers for that to work - from your comments on this and other threads that seems to be the case. Many clubs are not so fortunate.

I spent the other evening with a couple of friends, professional musicians with a national, even an international, reputation. Whilst much of their repertoire could be considered "folk", most of their work is now in arts centres and arts festivals rather than in folk clubs. They told me they were embarrassed when people turned up to see them in a folk club and had to sit through poor-quality and amateurish floorspots. This is all too prevalent, does nothing to promote folk music, and risks driving away those people who do want to hear good folk music.