How do you KILL an immortal???

This one is really interesting matter, at least in theory.

I don't know what your reaction on reading this, but I will put him in a particle accelerator. The mass will be converted to energy by the equivalence of E=mc^2, and atom by atom, the matter will be destroyed irrevocably. Since nothing can replace the matter that has been lost forever, the person would eventually be erased from existence. And the energy can light up a city for an YEAR.