Dyson Sphere

[DavidS]

My mind keeps going back to Phil Plait's explanation for why it isn't realistic to blow up a planet, the way it is depicted in science fiction movies. You have to imagine how much energy it would take to accelerate a single rock to escape velocity. You use this much energy to fling a rock skyward. Then you repeat until the planet is gone.

This site enumerates several options. None of the methods are what I'd call cheap (at least, not if time has any value at all).