I know for a fact that lethal force has no effect on the Joker. This monster has survived gunshots, explosions, helicopter crashes, plane crashes, car crashes, poison, drowning, the electric chair, freezing, fire, falling hundreds of feet, crowbars, massive brain injuries, and reality punches. He just won't die, anyone who tries to kill him is only putting their own lives at risk.
That's why we must forget about killing him, the only way to stop the Joker is to inflict the worst kind of pain possible on him, traumatizing him so much that he will never be able to pose a threat to anyone ever again. And only one man can do this. Batman! You must rape the Joker! It has to be you, it's the only way!