First of all its kinda sad for her to ask.
However in my experience most women who ask this question are insecure about the relationship. They don't necessarily need to hear those words. They just need to see some more interest on your part which is natural if you are playing the DJ game.
Try this.
Roll your eyes and try to make a joke out of the situation.
Laugh and tell her, "Well I used to, but not anymore."
She'll most likely ask why not.
Then say, "Oh I just change my mind from time to time. If I fall in love with you again I let you know as long as you don't ask first."
Shrug it off like she's being silly and that will keep her off your back for awhile. If she's really persistant then just tell her firmly that you're not going to tell her you love her if she asks you to.