For myself, I always ignore it -- having gone through a divorce from a social circle that LOVES drama, I've definitely had my share of ludicrous back-stabbing. If someone asks me directly about the rumor, I'll calmly give my side of it and then drop it. When I do give my side, I assume that others won't believe me. I also resolve not to care what they believe, and continue living my life in a way that I know is right for me.
Remember that most rumor-mongers are baiting you, trying to start trouble. If you respond in an emotionally charged way, they just got the reward they want and the crap will continue. If you refuse to play, they'll eventually get bored and go on to someone they can stir up.