Do I have to destroy the entire planet, or will it count rendering it unsuitable for life, or, even more limited, human life? Remaining within present day physics and completely destroying a planet is extremely difficult - even making the sun go nova isn't guaranteed to vaporise enough of it and push the vapour far enough out that the inherent gravitational force can't pull it back to something that is recognisably a planet. Grey goo won't do it - if you look at it right, life is already self-replicating nanoware, and it hasn't converted more than a tiny fraction of the surface into itself in several billion years of existance, ignoring the great majority of mass in the mantle and the core. Energy restrictions, mainly, plus finding enough of the right king of matter as building blocks for the next generation. Equally, generating a quantum black hole (in CERN) and capturing it in the heart of the planet, where it absorbs all the mass around it, getting steadily more massive and more voracious takes hundreds of thousands of years to work, even assuming Stephen Hawking is wrong about micro singularities evaporating - certainly not at the rate Niven's 'The hole man' functions. Explosions, even anti-matter explosions, won't scatter enough of the mass to stop it infalling back as a planet. No collision is going to do more than change its orbit, and unless the new path intersects the sun or, stretching a point, Jupiter, there's still going to be a planet there, albeit fractionated.
On the other hand, blowing all the atmosphere off, boiling away the oceans, fracturing the crust for volcanic effects Hollywood would love, all that's far more practical. Making the place uninhabitable is probably within our present technological ability. Making it impossible for humanity to survive is almost certainly possible for us, if you don't mind having the battered wreck of a planet still orbiting several million years down the line and available for forensic analysis.
