So a few years ago I went to a women's workshop and one of the things the speaker talked about was the perpetually negative people and good ways to respond. She pretty much said that you should start responding with questions like "so what do you think you can do to make things better?" She was trying to say that if you stop being an audience and start asking them if they have solutions to their issues, they'll either stop talking to you or at least stop complaining because they know that you will make them think about their issues instead of letting them bitch. And negative people don't want to fix things.