Perhaps it's not possible, but... I thought of a way "curing" a Holistic Spawn might be accomplished. I do, however, think that it would cause
serious problems with gameplay - something about switching a species of a character seems like it would mess up the civs they are aligned to... Or, rather, if they're already enemeis with Dwarven Civ #1 by simply having been a Spawn in the first place, would they somehow change back to a "Friendly" state again once they "turn back"?
Anyway. It is entirely possible to create a reaction at a workshop that creates a cloud of vapor. I used it to make dwarves bleed out all over the place once, just for the fun of it. (seriously, that fortress was the most bloodstained mess ever.) This cloud COULD conceivably (I think - my modding is very rusty) change a Holistic Spawn into a dwarf, and the method for doing so would be simple. This is how it would be done:
╔═╗
║¢║
╚╬╝
You put the workshop at the eastern or western side of this structure. Thus, dwarves won't have any issue with job cancellation by walking in front of the Spawn, which you drop from above via a trapdoor. The hatch cover you see in the image remains locked until the dwarf is converted. The vapor can circle around the sides and into the area via the fortification, the creature is converted, and the problem is solved.
HOWEVER. I'm pretty sure this requires a creature that can actually breathe, and that creature is
not a holistic spawn.
So... you'd pretty much have to change the raws for this to work, but it's a neat idea, anyway.
I'm kinda iffy about the idea of a spawnitis cure, too, though. It makes them a lot less threatening.