There is always the little white lie approach. You could say to her "you're very sweet but I'm seeing someone" or "You seem like a nice person but I'm more interested in someone (insert qualifier here that she does not have)", or something like that. Let's say she's 21. Just say you are really more interested in someone closer to your own age. If she flirts and asks why, tell her you really want someone with similar years life experience as you. Something she can't overcome.
It's less in what you say than how you deliver it. I'm the same way frankly. If someone is not my cup of tea I am very sweet about it but I smile and tell them gently. I make it a habit never to discourage someone from approaching me as I wish to reinforce the confidence to approach someone else (and I think being ugly to someone who has paid me the compliment of being attracted and making the approach is inconsiderate.)
I am gentle but direct myself to handle this sort of thing. Maybe that is more your style if you don't want to tell a little white lie.
The thing that I will most often do is smile sweetly and say "You are so sweet to notice me. I'm sure you are a lovely person. I'm just not feeling it...but I really do appreciate the compliment" I am caring and genuine when I say this to a man I am not interested in. Men have complimented the way I handle this repeatedly and some have told me that they appreciated my honestly and the way I treated them with kindness, courtesy and respect. It's still a rejection. But it's a gentle rejection that appreciates the other person's humanity.
