This thread subject deeply touches a nerve in me.
The occasion of non aware, non invited "musical" cretins parisitic assistance happens all to often and I consider such to be even beyond being very rude.
When I can see it coming, I first state, "I don't need any help". "Thank you."
Astounding to me is that this gentle approach of mine sometimes fails.
My second attempt to retreive my musical style involves a rapid unexpected retaliatory attack with side cutters! (The same wire cutters I carry to trim the length of newly installed strings.)
The desired result/effect I find is very satisfying to many of the previously uncomfortable dedicated attentive listeners who choose to share their reflections of the event thereafter. :)
I've even had to resort to this means before large audiences and while on broadcast radio.
Nuff sed! MT