What to do about this Facebook 'friend' situation?

[Mrs M]

I worked with a girl six years ago, then moved across the other side of the country (we were facebook friends at the time).
I didn't really want her as a facebook friend in the first place, but added her to be polite as we worked together.

Anyways, about 2 months ago I had a spring clean of friends list, and took out all the people I had not seen or spoken to in those six years, old work colleagues etc.
Now I find a friend request from her and I don't know what to do! I don't want to hurt her feelings, but I don't want to be friends with her, I am never going to see her again and we haven't spoken in six years....what do I do??