http://ned.ucam.org/~sdh31/misc/geocide/index.html
Here is a sample of one idea:
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.
A brief analysis of the size of the object required can be found here. Falling at the minimal impact velocity of 11 kilometres per second and assuming zero energy loss to heat and other energy forms, the cue ball would have to have roughly 60% of the mass of the Earth. Mars, the next planet out, "weighs" in at about 11% of Earth's mass, while Venus, the next planet in and also the nearest to Earth, has about 81%. Assuming that we would fire our cue ball into Earth at much greater than 11km/s (I'm thinking more like 50km/s), either of these would make great possibilities.
Obviously a smaller rock would do the job, you just need to fire it faster. Taking mass dilation into account, a 5,000,000,000,000-tonne asteroid at 90% of light speed would do just as well. See the Guide to moving Earth for useful information on manoeuvring big hunks of rock across interplanetary distances.
Earth's final resting place: a variety of roughly Moon-sized chunks of rock, scattered haphazardly across the greater Solar System.
Feasibility rating: 7/10. Pretty plausible.
Earliest feasible completion date: AD 2500, maybe?
Source: This method suggested by Andy Kirkpatrick
Comments: Earth is believed to have been hit by an object the size of Mars at some point in the distant past before its surface cooled. This titanic collision resulted in... the Moon. You can download a simulated video of the impact from this page. While the Mars-sized object in question obviously didn't hit Earth nearly as hard as we're proposing with this method, this does serve as a proof of concept.
