Set spawn point for a specific player

I made a server and me and one of my friends want to do a prank on another friend, because he stole all my enchanted diamond armor.

The Prank

I want to set a player's spawn point in a pit of lava, how would I do this without affecting anyone else's spawn point?

This can be done with the /spawnpoint command. The syntax is /spawnpoint <player> <x> <y> <z>. You need to set this point to be over the lava pit, not inside it, since the player can only spawn in the air or on a solid block (not in lava).

There are a few problems with this prank, though. First of all, if the player in question sleeps in a bed before dying then the prank is ruined.

Second of all, there is the fundamental problem that this prank can become virtually inescapable, and cause an infinite death loop. This is griefing. In the event that it does work (which it might not for the reasons stated above), the player in question would without a doubt get infinitely frustrated, rage quit, and never come back.

Furthermore, all of the player's items would despawn eventually. So if they happened to have your enchanted diamond armor on them at the time, that'd be gone.

What I would do? Use /ban <player> or /whitelist remove <player> instead, or at the very least, let them know that that sort of behavior isn't acceptable.