Shadow Sword (Yu Yu Hakusho); possesses an antidote that reverts its Conversion Slash
Moon Pool (H2O: Just Add Water/Mako Mermaids); can turn merpeople back into humans and vice versa
Mystic Falls (The Vampire Diaries); temporarily surrounded by a magic-negating field that could revert vampires and hybrids to their original state
As the "lump" anti-elemental, Lumpy Space Princess (Adventure Time) was able to single-handedly undo the conversion effects of the four elementals.
After several failed attempts, Batman (The Batman vs Dracula) successfully created a cure for vampirism, allowing him to revert all the people Dracula converted back to their original selves with no memory of what occurred.
The Omnitrix (Ben 10) can repair genetic damage such as DNAlien mutation, restoring the victim's DNA to its original state.
After scanning the Omnitrix's ability to repair genetic damage, Cooper Daniels (Ben 10: Alien Force) was able to create rayguns...
...to accomplish the same effect.
The Moon Pool (H2O: Just Add Water) can turn merpeople back into humans, and vice versa.
Peter Parker/Spider-Man's (Marvel Comics) immune system can fight off a vampire infection and revert back to normal thanks to his Radioactive Blood killing the vampiric enzymes that cause the transformation.
Anti-Venom (Marvel Comics) curing the Spider-Virus, returning the infected to normal.