Quote Originally Posted by Ziegander View Post
For me this is just a case of where fluff trumps mechanics. The spellthief is a thief. He's stealing your knowledge. If you still have it, then he didn't actually steal it from you. Fortunately D&D isn't a pvp game, and nobody actually cares if the spellthief steals all of the spells of an NPC Sorcerer. But yeah, if you lack a spell learning mechanism and a spellthief steals your spell knowledge you basically just get rek'd and cry in the corner.
I would at least put in some mechanism that allows a Spells Known caster to replace the spell with a different spell at some point during later downtime. Or allow the loss of the spell to be cured by some kind of powerful restorative magic of some sort. Something to make recovery possible, but impractical in the moment that they have stolen it.

There is no precedent for such complete and irrevocable destruction of character abilities, and I do not think there is a good cause to arbitrarily penalize anyone playing a Spells Known caster for their choice of class.

I get the fluff is important. Disable their access to that spell, because it was stolen, sure. But let them get the Spell Known Slot (at least) back somehow.