"Softer baby, softer...I'm a sensitive lady..." with a smile and a wink or whatever, at first...because as previous posters have suggested, most guys really just don't get that attacking the vulva like that is painful and possibly cystitis inducing. I also use a similar line on sometimes on clients who suck my nipples - "...oooh just around the nipples honey, I'm sooo sensitive, look how hard they are getting" etc etc. Most of the time I like my nipples being sucked but I get the odd guy that either does it too hard, or (mostly if I find them particularly unappealing for whatever reason) it just weirds me out. I think its because I usually find it pleasurable so when I don't like the guy very much it feels a little incongrous and bizarre.
I digress. If he doesn't slow down, or move his fingers to less sensitive bits then be really firm and say what they are doing is causing pain and discomfort. If they still persist its pants and socks on and out of the door sans refund, because either he is ignorant enough to believe that you don't know your own mind or he wants to hurt you.
But again, a lot of it is because they have been porno-grammed.