I've only seen people get off voluntarily - for example once a guy objected to sitting next to a large passenger. The FA took him aside (just in front of my seat) and said, this is a full flight, it's your choice if you'd like to be re-accommodated later. He got off. Actually he pitched kind of a fit & said some ugly things about the other pax, fortunately out of earshot of the other guy. My parents were once on a flight where a couple of drunk pax were arguing during the boarding process, male threw a punch at his female companion, they were tossed.

A friend who works for DL (first 12 or so years in customer-facing positions) once estimated he had taken 200+ people off flights (including meeting flights w Atlanta PD and some denied boarding situations along the way). Chemical impairment of some sort would be a common theme. The key to taking someone off a plane, he would tell you, is to not have the discussion about it on the plane. Quietly ask them to join you in the jetway to discuss the issue in private...then quickly shut the door behind them. And never, ever throw your briefcase at a gate agent.