Stab him, poison him, wave a magic wand at him, throw him into the sun Superman will always come back. Or, if you're the Time-Trapper, you could essentially just wait it out until it's the end of his life and then he'll just die of natural causes. That's the long and the short of Even A Superman Dies! from 1970's Action Comics #387. Superman, already knocking on a bit in this future-set story, goes to break some astronauts out of time bubbles that send them reeling through history. He accidentally enters a defective one booby-trapped by the Time-Trapper, which makes him age to about a million. Unfortunately, being old as balls isn't Superman's only issue. There's also the fact that he's alone in the universe save for a huge cosmic spike Lex Luthor made before he died, powered by evil psyche-energy, which hunts the elderly Man Of Steel down and puts him in his place, because if he fought too hard against it he'd break his hip probably.
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/