What should I say to a guy who says "I'm also a catgirl!"

I'm 100% not interested in him, but he keeps insisting he's also a catgirl and I'm a futa, and that we should fuck. He messages me every day, and I just want to shut him up. I'm considering the nuclear approach right now, but "I'm 100% not interested, I'm probably just going to block you and never talk to you again" seems unkind.