Attendees at any kind of session/singalong/whatever should resonably expect an enjoyable time playing together with other *musicians.* While it's unreasonable to expect all said musicians to be equally skilled, it's *not* unreasonable to expect totally UN-musical noisemakers to somehow be constrained, if not excluded.

It's also not at all unreasonable to expect beginners -- and all players unfamiliar with a given selection -- to self-censor and back off when appropriate.

How to achieve this aim peacefully and tactfully -- well, that's a problem and always will be. These problems have to be solved on the spot, on a case-by-case basis. I doubt we'll be able to come up with a solution that'll work in every situation.