"It is totally unfair of club organisers to put the onus on singers by suggesting that they should have to "ask their listeners not to join in"

I totally agree with that. In our club the problem is not so much with harmonising as much (which at least tends to be quite good) but with percussionists. Not the real percussionists but with ones who started on percussion as they feel they should be doing something and have that anyone can bang a drum attitude. Several months ago I had practised one of my own songs with a female friend and when we tried it at the club it was totally spoiled by a woman who'd bought a cajon and was going to bloody use it whenever she could. The song stops half way through then restarts at a much slower tempo then shifts back to the original. She started banging on the drum and made no attempt to watch and work out what we were doing etc and it ended up a real mess. We hadn't asked her to join in!

In our club you can have someone doing something quite rousing where participation is wanted. However if you are going up after that and want to do something on your own then you really now have to say "please don't join in on this" which is always a bit awkward. Much better that people only join in if asked. Especially as after our open mic we go to the pub for a free for all pub session afterwards where everyone can play all night.