The thing is, that's kind of a misnomer. I've seen this question many times, and it's a really misleading one- the actual answer is you don't 'kill' something that's already dead. You *destroy* it.
'Kill' means to 'make stop living'. If it's by some standard no longer living, you cannot make it live less. However, as many wise people here have mentioned, you can dismember, mangle, hack, and otherwise remove ability from things you can't technically 'kill'.
Frankly, Jason Voorhees only got as far as he did because the vast majority of his victims were unarmed idiots. Every time someone actually took him on with a modicum of intelligence, he lost badly.
Jason:
Smart Victim: "...and drop the net."
Jason:
Smart Victim waves steamroller forward "okay, bring it up...yes that's good...okay now back up over him...and forward...okay stop."
Smart Victim signals to front-end loader. "Okay, scoop his mangled carcass and some dirt into that pit over there."
Smart Victim signals to drop concrete
Once concrete sets, block of mangled Jason is dropped into Marianas Trench, there to wish he had powers other than regeneration and stabbing, because both are useless while encased in a 5 ton concrete block 2 miles beneath the ocean.
