Refer them to this thread, print it out and hand it to them, saying this could help them.When they come again, if they are below par, tell them they have either not read the thread or that they have missed the point.Recomend them to another venue.
At my club, small 'm' it by invitation or audition a I have to keep the punters happy first and foremost.Good luck .However a session or singaround is a different kettle of fish, and fish smells when its off.