Go inside, start burn on CD, get boombox, retrieve freshly burned CD, go outside, plug in Boombox, plunk it down next to perp and start playing your freshly burned Britney Spears CD circa 1998 or so at earsplitting levels while dancing with an improvised miniskirt on. If that doesn't make him leave I think you're out of luck.
Seriously though, when you're dealing with crazy, you just have to make them think you're crazier.
That situation would never happen to me because of how my property is set up, fenced, dogs inside that are not friendly, but, since this is really just an exercise in thinking outside the box...
I have a Daisy BB pistol, chrome, can't tell it's not real at night. I'd walk up to the door opposite him, open door, toss it inside to the floorboard or seat beside him. Grunt "throwaway gun". Slam the door. Walk around to the other side, pull my piece and wait.
Note I'm not advocating throwaway guns or giving him a fake gun so you can shoot him. Not at all. We're just thinking outside the box.
What I am doing is guessing that someone cool enough to know that you can't shoot them for what they are doing, will know that the situation dynamics have changed dramatically now that they think I put a gun in the car with them. They will know that from the legal standpoint, you can shoot an armed robber. I'd expect them to get a whole lot more cooperative in a hurry.
They might ask to give themselves up, they might bolt and take off, or they might toss the gun out the window and tell you to quit interrupting them you're disturbing their concentration. If you have the right persona though, you could throw them off their game.
That's just thinking outside the box though. It would be better to not confront at all. I'd stay inside, maybe ban supergas for something to do, he's kind of annoying.