Send them through a portal into a Mystcraft ocean age filled with Resonant Ender, meteors, and white decay. And destroy the entrance stone platform so they will never stop teleporting. Then send zombies in after them. And squids. Dozens of squids. Then up in the sky build an emerald status of a villager and set off a nuke.
Alternatively one could have them navigate a maze or labyrinth in the twilight forest to get away from zombies, but at the end they are turned into emeralds by an autonomous activator wielding a capitalist-enchanted etheric sword.
Stick them all in safari nets and send them endlessly looping through an overcomplicated LP system, comprising item cannons, itemducts, automatic exports, minecarts, laser sharks, and squids. Oh and make sure the minecarts go down a roller-coaster-style track that takes them through a mob farm.
Also one could place them in minecarts and send them along with a boring train to supervise. That would be quite boring.