Nine pages of posts say that plenty of people are like this .
There's a post in here somewhere that suggests painting them with that anti-nailbiting stuff that's supposed to taste awful or spraying them with deodorant - it would only really work on the slobberers, chewers and biters but it's a start?
I can always tell when a guy has sprayed Lynx all over his chest because his nipples taste really gross and my mouth goes all puckery. So try spraying perfume directly onto them if you want to discourage it, but that won't work for guys who just want to pinch/tweak them.
(In all honesty I have the opposite problem: I can feel hardly anything in my nipples. If a guy really bites them then I can feel it. But if they just touch them, or anything on my boob, unless I'm actually looking at them I have no idea they're even touching me.)
I think just putting "I don't allow nipple play" on your profile should weed out a lot of guys, and maybe if it's a common problem state this when a guy books. "I provide kissing, oral without condom, light fingering, and full sex with condom. I don't allow nipple play or anal" should do the trick.