-PalookaLord - A bullet through the brain. If their minds have become so damaged to associate themselves with something so unmistakably inhuman, they're likely too far gone for any kind of help. Killing them would be a kindness.
-DevilMayCrye-Seeing all your loved ones either killed or trying to bite you. Either lure the Quisling into a room filled with zombies, or as PalookaLord said "A bullet through the brain."
-Whtwlf: If my encampment had the resources, we'd probably try to restrain and psycologically evaluate the quisling. If there was a possibility of recovery from the condition then we'd try it, if not, we'll probably have to kill them.
Renegade13: Restraint, evaluation, if they can recover help if not say good bye.Bullet to the head.

Quickrace89: The condition could be fixed... Hypnotism can put you into that state, and I bet hypnotism could take you out. Another way would be to heavily sedate said Quisling, put them on fluid and vitamin support via a drip, and let them sleep for a week or two. My money's on the fact that they would still have dreams. And, there's a lot of very evoking things that we humans have. Pain. Alcohol. Drugs (LSD hallucinations might re-awaken self-awareness, or drugs like Heroin and Methamphetamines which release all the happy endorphins in the brain, and jolt the sh*t out of the user). Epi-Pens (a shot of adrenalin might do the trick, as would the jolt). Sexual stimulation (Let's face it, that's pretty damn evoking. I don't think I need to expand on that.) Basically, you're trying to remind them that they're human. And, being the most intelligent of all the sentinent creatures, there's a hell of a lot that can be used to remind us.