If I want to sing something unaccompanied, I make a big production of putting my own guitar aside. Most people have figured out that, if I'm not playing, they shouldn't either. If people still seem to be poised to jump in, I launch into an introduction that explains that people were often encouraged to "give us a song" while the musicians were taking a break and having their "tea". That usually gets the message through. If that doesn't work, a simple "please don't play on this song" makes it crystal clear.

Regards,
Maryanne