Ha! That's worth a few rare pepes.
He's just a persistent asshole. The board owner doesn't ban him because this is a haven of free speech. Having guys like that around is one of the prices we pay to enjoy free speech. He's exercising HIS freedom of speech. You may not like it, but you have two choices: ignore it, or filter it.
Or, I guess you could spend a lot of time complaining about it, but that time is wasted, you will not convince him to stop. If you want to post somewhere that's censored for your comfort, try Reddit.