I usually don't say anything unless they include me or ask my opinion. I spend hours with customers at my full time job and they say all sorts of stuff that I just let slide because it's not worth it. If it is directed at me, which has happened, I offer to stop the ride and let them out of my car. I usually say something like, "Are you speaking to me like that?" Usually, at least one of them has the sense to tell the bigmouth to knock it off.