User blog comment:FairyRaven/Fairy Tail Chapter 416: Tartarus Arc: Final Part, Review and Commentary/@comment-203.215.121.122-20150121234710/@comment-7418318-20150125092704

It depends on how he was created I suppose.

If he is a Human or other being that was turned into a Demon, they can just prevent him from being turned.

If he was created as a Demon, it'd probably take an ancient ritual, artifact or both to purge the Demonic from him and transform him into a Human.

Or come to think of it, they could do something like what happened to the Galuna Island Demons. They could lock up his powers, alter his memory and he could live out the rest of his life believing he was Human... Maybe that is exactly what Igneel (or maybe a combination of Igneel and time-traveling FT members) did to Natsu in the past.