Masked 'cat - I've been on the other end of that dilemma recently.
We were at a fairly lively song-dominated session, in the "taking turns" mode.
Toward the end of the evening I took my courage in both hands and decided to sing a song I had written recently. It relies on simplicity and a particular rhythm, and had never been heard in public before. I specifically asked a friend of mine to drum for me (he is very experienced & I knew he'd suss out the rhythm & then enhance it). This was taken as a general invitation to join in - so I shortly had 3 more guitars ad-libbing along (I couldn't hear my guitar - so I can only assume they were following eachother!).
I had to force my voice to get over the top of that lot, and by the end there were also a fiddle and a whistle....
It was really great that everyone joined in & enjoyed themselves - but I was giggling inside because I just wanted to stand up and say "I don't know what the f*** you're playing, but I'm trying to play this......". A friend of mine said she was giggling because I looked so furious - but in fact it was concentration, trying to place my melody line where I wanted it in amongst all the clashing notes.
I am still bemused by the whole thing, because everyone clearly thought they were helping and they all enjoyed it immensely - but how on earth did they think that they knew what to do to accompany a song they'd never heard before, when there was no way they could actually hear my guitar over theirs? I had to shout out "its going to change now" just before the middle 8 thing.
Sensitive accompaniment is fantastic, and a free-for-all on wellknown songs can be great. But this was just bizarre!!
Anyway - to get to the point..... is there anything I could have done or said without coming across like a control-freak prima donna? And anyway - would it be the right thing to do anyway? At the end of the day, there will be other occasions where I'll sing that song the way I want to present it - so there's nothing lost.
I think I'd come down on the side that since its a session situation then everyone can join in, and if it has a negative effect then it would have to be persistently VERY negative before someone should be tactfully spoken to about it. I felt that the accompaniment described above had a negative effect - but that was only relative to what I had intended, and in fact everyone was enjoying it, so that was a very subjective assessment. And if I had discouraged them on that song, I'd risk them never joining in with me again - which would be stupid!
As to how to deal with it if someone does try to stop you - I'd suggest nod graciously and accept it for that tune. Think carefully about why they had said it, and make any changes you think are reasonable. Maybe talk to a few other people present to get they perspective. Then start playing again quietly, and proceed from there.

Kris