Gargamel is an idiot.
He makes magic with ground up Smurfs.
UNless he routinely catches a couple Smurfs to ground up, and if he was a serial killer of Smurfs, the village would destroy the dirty hermit while he's sleeping, which means that he's buying ground up Smurfs from "the shop" (Yes, the CIA. Shut up.) harvested from other villages, or he's filtrating Smurf biowaste from the watertable if he wasn't out right found a couple abandoned mass latrines. But if you can make make magic out of Smurf poo, then magic comes from what Smurfs eat and not something unique to their anatomy.
Ground down smurfberries will give you probably more magic than a ground down Smurf.
Unless, a Smurf is a biological processing factory that turns smurf berries into Magic fuel.
Obviously if the Smurfs were to change their diet, their ground up limbs could no longer underpin magic and spells.
They should eat Gargamel.
Ozrael probably tastes better.
But the hunter becomes the hunted.
Can you imagine if they don't kill Gargamel at first, and just cut chunks off him to last the village weeks as Greedy Smurf explains the dishes that he's going to make out of the old wizard with extreme clarity and detail.
Of course if Gargamel had been eating Smurfs and then Smurfs started eating Gargamel, wouldn't that be one degree of cannibalism?