Are there any official rules for psychic powers or rituals capable of exorcising daemons from possessed characters? I can't seem to find any. Thanks!

Aside from the aftereffects/Elite Advance associated with exorcism in [Dark Heresy - The Radical's Handbook], no actual rules for performing exorcisms come to mind. Maybe someone else has more info for you.

Fate Power "Purge the Unclean" from the Inquisitor's Handbook (p.51)

Spoiler alert

In the Dark Heresy Church of the Damned - Book 2 of The Apostasy Gambit p 22 the pc's do a exorcism and this would be a excellent way to do such a complicated issue.