Guys, guys... you are obviously misreading the intention of the OP here. He wants funny ways to
destroy the Earth, not simply render it sterile or make life unpleasant for Homo Sapiens.
Take for instance grey goo. Sure, you've converted all available mass into copies of the goo, but the planet is still physically
there. And, heck, depending on how resistant the goo is to temperature and pressure, the goo layer probably only extends down a few miles, below which the fundamental structure of the planet is unaltered.
Now, if you were to engineer your goo with tiny rockets and enough reaction mass to reach escape velocity,
then you could do something like destroy the Earth.
Or, there's always my personal favorite method -
crash a gas giant into it.