If she wants to put on a cover, I'm not going to stop her (and I'm certainly not going to walk out of the session). I'll just consider it a disappointing session. But it just doesn't happen to me that often.
I can think of only one time when I expressly tried to override a prostitute's stated limits. It was a session through an agency I'd used several times before. All the women I saw through the agency had kissed. I finally went into their highest price category, more than I'd ever spent for an hour or have ever spent since (I'm not even gonna tell you what it cost; it embarrasses even me). The woman (who evidently was very new to this stuff) and I had a very pleasant conversation -- we seemed to be clicking -- and I leaned forward to kiss her.
HER: You can't do that.
ME: What do you mean I can't do that?
HER: Nobody kisses. Everybody knows that.
ME: That's not true. In my experience, most people kiss.
HER: Really?
ME: Yeah. And frankly, for $X,XXX an hour, I expect it.
HER: Oh. OK. (Leans over. Grabs me. Commences very deep DFKing. Rest of session goes the way I like it.)