Hmm. Do you like her as a friend? or you simply dont like
her.
Yah, just say it straight. The thing about saying it straight is
people tend to associate it with being callous. It isn't and
doesn't need to be unless you make it so. You can tell her gently
and kindly that you treat her just as a friend, Check with her if
your actions may have misled her otherwise and say sorry in advance
if you did. Tell her it is because you treasure her as a friend
that you are doing this, and you find it very difficult to continue
the friendship if she has 'expectations' from you and you know, the
standard waffle about her deserving someone better/more
suitable.
Then humbly ask HER if she still treat you like a friend despite
you saying all this.
After that, the ball is in her court. Even if she doesn't stop her
advances, you have absolved blame for uself lah.