What's the easiest way for your stereotypical aliens to destroy civilization from orbit? I'm taking "destroy civilization" to mean the destruction of all large-scale forms of governance, but bonus points for extinction.
Criteria: Either cheapest or least energy consumption. The aliens have access to anything that's roughly possible for a Kardashev II civilization. The aliens use a magic-plot-teleporty device to get here, but everything else should be as hard-science as possible.