Well, sometimes people just don't ever get it and they need to be told to fuck off in very blunt words. I think that usually poisons the well and dries up further conversation. But I'm not sure if this is the case here. And that strategy comes with possibly other severe consequences. So I won't recommend it.

Another strategy would be to have someone else talk to them... You yourself seem to be getting nowhere. But maybe he listens to other people, or they're somehow more gifted to get through to people like him.

What also sometimes works (depending on circumstances) are large headphones. They might be part of your work anyways, if you're doing online-meetings in the office, or you are allowed to listen to music... Either do that and you can't hear him anyways, or just put them on all day and say "Huh?" 200 times a day and see if he picks up on it. Though, this might not work if he's stupid, as well. Or he might start tapping you on the shoulder and invade your privacy even more... Idk. But headphones have worked for me in various situations. Especially if they're big and noticeable.