Nothing- let them be the only asshole in the room, most people will instantly regret being rude. Walk away and let them live with their own harsh words.
"Oh I will. I'm gonna dig up your grandad and ride his femur like a banshee" Works better if they recenty lost someone and you claim your going to necrophile that person.
Be prepared to have your teeth knocked out with a random peice of street debris. Necrophilia against the recently deceased seems to be a touchy topic for most folks.
“You don’t need to tell me twice!” “Then turn around because I can’t do it if you’re looking” “I don’t need your permission to do what I’ve been doing since I was( how ever old you were when you started masturbating)”
Fuck me yourself, you coward.