How do you reject someone without hurting them?

You can't and its not your responsibility. Be gentle, unless she still doesn't stop then you have to step it up but you cannot avoid hurting someone, you can't try not to hurt someone at your own expense and pain is how we learn... If nobody has ever said anything to her before then she won't learn.
Its only bad to go out of your way to deliberately hurt someone and for no reason, that's not what your doing.

Find her a boyfriend, something else to distract her from you. Then quietly slip out of the back door and join the French Foreign Legion!

Tell her the truth as kindly as you can.

First you use her affection for you as leverage and get her into bdsm, and then you make her get turned on when you hurt her, and then you totally blow her off so that she gets a thrill as youre dumping her 😈😈😈😈