How big a circle? How many songs do they get to sing? Who is "in charge"? If they offer 4 out of 25 songs in a night, I would grin and bear it or speak privately with them questioning their agenda in a VERY friendly way. They don't seem to be breaking any rules. If the intros are becoming a problem and/or are downright offensive to some, I would have the moderater have a friendly word with them suggesting that they do not need to offer introductions. The music is self explanatory.

It is entirely possible that they love the music so much that they are unaware that they are offending some. It would be a shame if it got ugly and anyone "lost it" over music.

Another ploy: suggest a theme night. Murder ballads, fallen women, Bobby Burns night, etc. Help them develop some variety.