Explain nicely that this isn't that type of establishment

You figure the best way to approach this is by explaining things nicely. You resume your work on her low back and tell her "Well Mrs. Carter, I wouldn't say that the "happy ending" is standard practice at most Massage Therapy Practices, and it isn't anything we do here. In all honesty, if it was something I was caught doing, I could get fired for it. Did your friends tell you something differently about our practice, or do you think they were just having some fun with you?"

"Don't take that condescending tone with me young man. You talk to me like I don't know what a "happy ending" is? I come in here every week, pay you good money for you to stare at me, and rub my half naked body. Well my vagina is part of my body, and I want it rubbed. I don't want to hear about you losing your pathetic job, I'm not going to be talking about this with anyone. So what's it going to be, do you do your job, or do I find someone else who will?"