There are a lot of possible responses, like urging humility and greater respect for others or pointing out we have no idea if the driver is borrowing a relative's car, but I would say the one I feel most strongly about is that when a person is criticizing or considering a political group as a whole, there is a moral obligation to address the group's views in toto rather than dismissing them for particular positions that may be wrong or weak. So, the sovereign citizen movement, while admittedly based in part on various types of bullshit, is also based in part on notions that are harder to dismiss, like the view that the U.S. federal government is dominated by corrupt elitists who have lost any sense of obligation to the citizenry. So I wouldn't dismiss them entirely, again, even though some of their views are bullshit.