How do I tell someone, who's in love with me, to just go away?

Wow that's tough, try telling her almost exactly what you wrote here. Tell her when you said that you wanted to only be friends, that is just what you meant, nothing more; and that your feelings will never change. Tell her you said you wanted to be friends, because you don't hate her and didn't want to hurt her feelings. Make sure she knows that you think she is a nice person, but you will never like her that way.