The best thing you can do is ignore him. He's a narcissist - a reaction is what they want. They don't care whether it's negative or positive - as long as they get *something* from you, they will consider it a win and keep engaging. The only way to "win" is to completely starve them - aka, block them without responding.
Sorry you have to deal with this kind of nonsense. Perhaps this person will find some peace one day.
I'm currently gathering evidence of all their continued harassment over several usernames and I will present it to itch.io in order to have them banned from the site indefinitely. I encourage you to take screenshots of all of his antics and do the same.