Mark,
I have lost two long time gigs over the same issue this year. In my case I stopped playing and asked the offenders to be a little quieter. In one scenario I was dubbed to be rude and disrespectful although I was very polite in my request. What bothered me was those who came to hear the music were not allowed to do that and they would look around at the offenders and you could tell they were not happy about the situation. In both cases the staff was useless and it seemed scared to do the right thing. In hindsight it has not mattered to my gig schedule and I have moved on. Now what I do is simply turn up the amp and keep on going. If I get complaints I tell the AD the noise level required me to play louder. Like the lady said, play the gig, take your money, and don't worry about it.

Deane