Perhaps you need a pre-shoot agreement.
"No fiddling with my nipples, either on set or in post"
Being serious, I once shot a girl with two-tone nipples, which the magazine evened up in post before publishing.
I dont overly mind people touching up my nipples.. in the most non sexual way possible...
but in most they have been made really brown or alot darker. :/
I think a pre-shoot arrangment is probably going tobe the way forward lol