You can get help in therapy, particular cognitive behavior therapy. You can also start your own investigation by realizing that the smile is a mask to cover other feelings. So experiment, next time it happens, spend a moment observing yourself, identify the feelings, and work to figure out why you would be uncomfortable feeling, or expressing, those feelings.
Hey John, I thought I was the only one on the planet with this problem! I feel for ya, and know exactly how you feel. I'm intelligent, a college graduate, scratch golfer, but cant help from smiling when nervous. Typically in conversation. Makes no sense. I'm in control of every aspect of my life except this problem. Relationships with friends and family are struggling, work, etc. Everyone looks at me like i'm a complete moron. My future looks very bleak if I don't find a solution to this soon. Please write back with any ideas. Tempe