As has been hinted by other users, one very effective way to pursue this is to go after publishers of adult novels targeted to females, which are well-known to feature rape fantasies.
This should be pursued not from the payment processor angle mind you, but by asking these publishers publicly whether they support the rape of women through their published works. Let them then make a considered and nuanced argument in public in support of free speech in art. If the thirsty ones who buy these books by the cartful catch a hint that their favourite fantasy is being targeted by busybodies, they will make themselves heard in no uncertain terms.