If you don't like her, you don't need to feel bad about being honest with her. You don't have to be "nice" about it either. Make it clear that while you have nothing against her, you have no romantic interest in her, either. Tell her that if she can't respect the way you feel, you won't allow yourself to spend time with her or have contact with her. And the whole time, speak firmly, politely, and authoritatively; whatever you do, don't use the word "sorry", it weakens your position.
With all that said, just how much do you really not like this girl? Do you absolutely detest her, or simply not feel fond of her looks? If you're like a lot of aspies, including myself, you probably don't have girls interested in you very often. So why not date this girl, assuming you don't have any specific reasons against it (besides simply not liking her). As guys, we have the ability to date girls without feeling attracted to them (as part of a millions-of-years-old evolutionary mechanism designed to "spread the seed"), so take advantage of that ability, and get some experience.