> Nearly all of those ads promise “$19 service,” or thereabouts, a suspiciously low sum, given that “locksmith”-related ads cost about $30 or so per click, depending on the area.

There's the weapon against them right there. If you're a crusader, don't be a volunteer mapper for Google. Find the fake listings (drive there and look, or street view?) and then click on them, costing the scammers every time. I imagine clicks could be automated once discovered? Or at least shared with other volunteers who can click.