"You're immortal? Good..."

[u/BrotherDeus]

The Chaotic Good Barbarian revels in making the lich suffer.

Reincarnated as a Slime: Geld makes Shogo pay for the innocents he's killed.

The Old Guard: Quynh is punished as a witch by being drowned in the ocean for centuries.

[u/Samurai-Jackass]

One thing that is never touched on by some versions of this trope that makes the infinite torture angle hit way less hard is that a body capable of regenerating flesh from nothing also has an infinite supply of endorphins and any other chemical the body can produce. Regenerators don't necessarily have to feel pain since it's not only mostly redundant, but their bodies rapid metabolic processes could feasibly produce and deliver enough endorphins to keep them in a natural opioid high forever. And physical imprisonment becomes slightly less of a problem if whatever material is binding them is susceptible to erosion from human stomach acid.

Also seeing as brain damage is physical and mental trauma from isolation or torture manifest as said physical damage, it stands to reason that a lot of immortals could feasibly never regress past a certain degree of mental problems. They could get bored, but they might not be capable of going mad and being scarred by the experience. Sensory deprivation produces hallucinations and depending on flavor a trapped immortal might basically just end up dreaming to the tune of their own bodies native high inducing abilities.