I thought of this scenario a week or so ago because of a strange experience getting on the highway. I tend to drive fairly fast, and as I was accelerating down the access road I passed some cars at a fairly high rate of speed (three lane access road, I wasn't even close to them). However, one of the cars apparently didn't like that (I guess), so he got right behind me as I was getting on the ramp into the highway, flashing his brights at me (it was at night). There was a truck ahead of me, and when the truck pulled out of the merge lane I simply accelerated because I had plenty of space left to get in front of him before the lane ended. The car behind me decided he was going to do the same thing, though he didn't have the space (nor the acceleration to keep up). So he ends up driving on the shoulder after the lane ends and then falling in behind me. I'm a little concerned now, because he's crossed the line from just being someone who's possibly annoyed with my driving to being someone who's taking some really dumb risks. So I keep a close eye on him and he quickly pulls up alongside of me and slams on his brakes so that he's going the same speed as me. I look over to make sure he did it on purpose (and to see if he's doing anything else, like waving a gun). As soon as I can tell he is looking at me (and since I know there's no one behind me at this point) I press on the brakes to get him in front of me. He pulls into my lane and starts flashing his brake lights, but not actually slowing down. I simply coast to let him pull away some more and then resume driving. For a couple more miles he kept changing lanes to stay in front of me. Eventually he took an exit, and after watching the following entrance to make sure he wasn't just trying to trick me, I went back to my normal driving.
Still not sure what that was all about, but I was thinking at the time that he might have pulled a gun and started shooting it at me. I agree with the other posters that disengaging is the right way to go. However, if I did end up pinned in somehow, I would stop the car, open the window and door, and use the door for cover to figure out what he was going to do next. It was definitely one of those few times that I have been carrying that I was really glad I was, which makes all the other times that it's primarily a hassle worth it.