It's probably not that you have some dumb look on your face, but you may be close. You may look like your at peace. When I don't want to be bothered, like walking down a city street full of people looking for opportunity, I put myself into a state of concentration and intensity. While maintaining awareness of everything peripherally, I never acknowledge anybody's presence by the slightest outward gesture that I'm aware they even exist. Low hanging fruit gets picked first. Where I spend most of my time I don't need to do that and enjoy talking to strangers.