There is nothing wrong with looking after yourself.
It sounds like the guys that think you are in a relationship think that girls that are single, don't take pride in themselves which in itself is sad.
I imagine that you hold yourself up well. Feel confident in yourself, your life where it is going and perhaps put an air out there of being un-available and un-attainable.
Which is why people think you are taken.
I agree, smile. When someone smiles, someone else smiles back at them, with them and it gives an opening for communication because what ever they thought, goes out the window, that person smiled at me so "she" must be ok to talk to, I'll give it a go.
It's funny, usually the guys that are idiots talk to any girl
Sorry guys, I know most aren't .... just saying