Its interesting that i used to have (ands till sort of) a similar thing. Im a very friendly guy, get along with people easy, and it is beacuse i smile and show interest t what people say. So i noticed that even when i was not interested i would smile as if i was, so i had a lot of unasked attention, which made me tired of it. So i worked with my head first.
So, how do you feel if you dont smile, when you are with people? to me it was kind of strange at first, cuz people thought that i wasnt ok becase i wasnt smiling as much. And what people are we talking about? strangers or relatives, or friends?
So you learned to smile, now because of your life's experience you will learn not to, so in a way you will naturally stop smiling when it si not appropriate. Even if its annoying, you will notice when it happens, and you yourself will shut it down. Just stop seeing it as a problem and relax bit about it so you will be able to learn "when to smile". Otherwise you'll stress yourself over i, and learning will be harder. How about this. Allow yourself to smile only when you greet that person. Breaking habits requires small steps, so unless you get a psychological trauma you're not going to stop doing it immediately. It also helps talking to close people that this happens to you with. When you tell them the illusion of the power of smiling fades because "they have found you out", so to speak,and smiling wont have an effect on them anymore.
hope this helps =]
PS. Don't worry about a psychological term, illness names often make the person identify with their problem, which is hindering for their self image. You simply have a life situation.