Is the person telling lies about you? Or is he/she passing on true information? S/he may simply be unable to distinguish between information that is o.k. for public consumption and information s/he should keep silent about.
Some people simply have no discretion, it is not necessarily their fault. It is unkind to tell this kind of person anything private.
Unfortunately, reality tv shows, celebrity gossip magazines and the like encourage people to think that gossip is normal. They treat the people in their lives the same as they do celebrities and speculate and gossip about their lives.