From the Slacker Astronomy extra podcast:
How to Destroy the Earth
7. Pulverized by impact with blunt instrument
You will need: a big heavy rock, something with a bit of a swing to it... perhaps Mars
Method: Criminal, really, that this blindingly obvious method was overlooked for so long. Essentially, anything can be destroyed if you hit it hard enough. ANYTHING. The concept is simple: find a really, really big asteroid or planet, accelerate it up to some dazzling speed, and smash it into Earth, preferably head-on but whatever you can manage. The result: an absolutely spectacular collision, resulting hopefully in Earth (and, most likely, our "cue ball" too) being pulverized out of existence - smashed into any number of large pieces which if the collision is hard enough should have enough energy to overcome their mutual gravity and drift away forever, never to coagulate back into a planet again.