The Canons of the Council in Trullo 60

Since the Apostle exclaims that he who cleaves to the Lord is one spirit, it is clear that he who is intimate with his [i.e. the Lord’s] enemy becomes one by his affinity with him. Therefore, those who pretend they are possessed by a devil and by their depravity of manners feign to manifest their form and appearance; it seems good by all means that they should be punished and that they should be subjected to afflictions and hardships of the same kind as those to which they who are truly demoniacally possessed are justly subjected with the intent of delivering them from the [work or rather] energy of the devil.

Notes.

Ancient Epitome of Canon LX.

Whoever shall pretend to be possessed by a devil, shall endure the penance of demoniacs.

Zonaras says in his scholion that even in his day people made the same claim to diabolical possession.