Unfortunately I am one of those people right now! Luckily I'm self aware and try to keep my b*tching to a minimum unless someone specifically asks about it. At this point, I know what I need to do to END the situation so I'm no longer in a position to have to b*tch and moan about it.
But the easiest way to shut up a chronic complainer is to simply tell them that you've heard enough. Most of them aren't aware they won't shut up about it, and once you inform them they're generally really good about it.