Have you tried talking to this person? I mean face-to-face. The important thing to establish is whether or not this person is doing it maliciously or if they are just a little bit oblivious to the fact that they are being inappropriate. I mean, if you tell someone something in confidence, specifically say DO NOT repeat this to anyone, and they blab it out, that's one thing. But, what if they are just not aware of whether or not that information was inappropriate to share? Some people just don't have as good a concept as others as to what is ok and not ok to talk about, and who it is ok to talk about those things with. Especially if this person may be from another program, where things may have been done very differently. Or, if they are the type of person who
Some people need things spelled out for them, literally... "XYZ is ok to talk about, ABC is not.... this is why..." Give this person a reason for why they are being inappropriate, how it is effecting your program, etc. Don't automatically write them off. They may just have had no concept of what they were doing, and will probably feel pretty bad once you point out to them that they are causing damage to others. Just lay it out in front of them- what is expected of them, who is privy to what information, etc- and let them know that you are seriously unhappy with what they are doing, and expect them to change their actions immediately.