How to politely end things with a FWB?

I've met up with this guy a few times, only when I'm absolutely bored out of my mind. He is terrible in the sex department (for me), we have zero attraction to each other, no romantic feels, we are the definition of friends with benefits. We talk about about him and his life mostly, but he doesn't take no for an answer. I've tried the ghosting when he msgs me, I never msg him first. I flat out told him no the last time I saw him. But Everytime I see him in person I do end up sleeping with him. Is it just a will power thing? I mean I just feel Soo bad seeing him Soo turned on and don't want to give him blue balls but it's not my responsibility. Ugggghhhhh