Imagine your worst enemy is unable to die, and will not listen to reason or give up. How would you defeat him/her? Feel free to be as over-the-top as you'd like!
Get an inane but catchy song stuck in his head. Won't kill him, but will distract him for some time. Whom the gods would destroy, they would first make mad.
I suppose, given that this is a fantasy, you could die and go to heaven, and he couldn't follow you there! Victory! Or you could put him onto a rocket and launch him into deep space.
Haha, wondered how long until someone suggested trapping them underground! I actually started thinking about this predicament while watching anime, and it turns out that's how they dealt with him! All of these are very clever, thank you all!
I believe that giving an immortal an extremely high overdose of anesthetic will put him in a long, long sleep. Brainwashing or cortrolling their minds is also nice. If youre really mean, cut him up into tiny pieces so that he won't be able to do anything
Agree with the chopping up routine. Should be quite effective. Drive him mad by giving him huge does of psychoactive drugs and/or syphilis. Paralyze his entire body and leave him in a dungeon. Put him in a cage with rabid dogs. Needless to say, even if he lives he's likely to be good amusement. I could go on but I don't want the white coats to get me yet.
Drop him in an unstable wormhole. Scatter his subatomic particles across a few billion cubic parsecs of space. Used it in Neverending (part 2)
Go back in time and kill their parents before said character is born. Note: this doesn't work if they're immortal too. Then it becomes a wasted journey, and their parents will probably kick your ass anyway.
Learn to understand my enemy, fall in love, get married, and then slowly change my enemy with the power of love! Alternatively, force my enemy to reincarnate until he or she turns out good.
Put him in a tickling machine and tell him you'd only release him if he agrees to be a good guy. Or, put him in an endless battle with Goku. Eventually, he'd give up.
Launch him into space and hope he gets stuck on a planet where he has to be a gladiator for all existence.
I'd warp his purpose to my own but do it so that he had the illusion of making these decisions on his own and think that he was bringing about my downfall. I'd do this so artfully that he would lose all sense of where he ends and I begin and eventually it would send him into madness.
Lol I had the same dilemma actually, except my immortal became immortal because he supposedly lost his soul - traded it for immortality, so he can't die cus you can't get at his soul. My solution - since been scrapped - was actually to return his soul to him, returning his body to its original normal, mortal state, and then he dies naturally due to the fact that he should've been dead long ago.