"OK guys. What is the best way to tell a performer who just turns up to a regular session in a pub, that he is utter crap?"

"If they're a pure beginner and not very good, then I think patience is essential"

Everybody has the right to get up and have a go - they may have been practicing for weeks at home and go to pieces in front of an audience. Why not have a quiet word with them and say something like "you seem to be having trouble with those chords - can I suggest a better way of playing them?" or you get the people who start too low because it sounded OK in their front room - again a word of advice like "It's not a problem if you want to stop and put the capo up a couple of frets - we've all been there"

I don't expect concert performances from people who come to my club - we're there to enjoy ourselves at the end of the day but it's good to see how people improve over time if you give them some encouragement and advice now and again.

Cheers
Mark