....although this is just MY point of view, and I know other providers' opinions differ: there is no "good" way to do this without potentially hurting the lady's feelings. And feelings lead to anger; anger leads to the DARK SIDE! (yes, I needed to quote Yoda lol)
Once a lady's dark side comes out, anything can happen, even with ladies who have no public history of BSC behavior.
Walking out, most especially if the lady's photos are recent & accurate, can bring out and/or highlight a woman's insecurities about herself and her appeal. Pretty much all people have such insecurities. The fact that we're professionals doesn't mean we can simply turn off our "emotion chip". (see what I did there?? both Star Wars and Star Trek TNG references in one post! lmao)
I use RS2K for the vast majority of my clients and there are definitely some who have reports on them that they've walked out with or without paying a cancellation fee. A popular move is when the client walks in then immediately says he "forgot the donation" in his car so he leaves to "go get it". He gone!! (now I'm mixing in baseball quotes! yikes!!)
There are reviews floating around where the reviewer states that he almost left at the beginning but later was happy he stayed, so first impressions aren't everything. I even have had near-walk-outs wind up becoming very regular repeat clients so you just never know.
Offering a cancellation fee, or even giving her the full donation (which, if she did not misrepresent herself in any way, yet you are just not attracted to her is the appropriate thing to do, IMO) will decrease the odds of the lady retaliating. But nothing can really eliminate that risk. Human beings are just so darn unpredictable!