I think I've got the answer now: John Constantine.
No, seriously. John will do it, and you don't even have to ask him to. Just hit Metropolis with a scourge of hell - best done by way of a botched demon summoning to allow Satan or Nergal or Calibraxis to walk around unhindered - and watch big blue go into one of his climactic battle scenes against said beastie. In walks John with a swagger and a cigarette, and he says to superman that he can send the hellish nasty back to where he came from.
So, the magus draws up a gateway to hell on the ground and says "Oi! Drag 'im over this!", implying that the demon should be tossed in and it'll disappear with nar'ly a flicker of fire and brimstone. Except that the demon fights tooth and nail against submission, and its supernatural strength keeps it from an easy win. Superman ends up having to literally drag it back into hell, ultimately getting himself sent there in a way that pisses John off.
This is not only a sure-fire way to kill Superman, but I bet that if somebody wrote it, it would sell. There is no sun in hell, and even if Supes had his powers down there, most demons are stronger in hell, the big ones nigh-unstoppable and Satan practically omnipotent...save for interference from god. But we know that even deus ex machina wouldn't write him out, since Superman's also a self-recriminating bastard and find himself deserving to be there for something.