Re. the spoiler item: I would accept that as a resolution to the plot, for that character only
. Not as a magical solution that suddenly banishes the boogeyman.
I think what I'm trying to say is that there's sort of a fundamental disconnect at work in movies like these. You've got immortal souls, immortal beings, afterlives, places of eternal punishment, all of which should be governed by distinctly metaphysical rules. (I'm not going to debate what those rules are for fear of starting a religious war!) Suffice it to say that belief should be a very strong part of those rules, and it's not unreasonable to suggest that a demonic villain can be defeated by a sincere and abiding belief
in Jesus, Allah, or whoever. (It may be unreasonable to show
it, given religious differences these days, but that's another story!)
But these movies rarely work that way. Despite the fact that demonic villains are implicitly or explicitly shown to follow metaphysical rules, they're almost always defeated by physical (or at least formulaic) means. Recite these magic words and it will banish him. Arrange the puzzle box in a certain configuration and it will banish him. Stick him with a magic sword/piece of cold iron/beam from a magic wand and it will banish him. Give the cursed coin to someone else and... well, it won't banish him, but he'll instantly forget about you and go after the other person instead.
The disconnect is showing a being of godlike power who doesn't abide by the rules of this world, yet he still somehow gets defeated by the rules of this world (and isn't too smart, either). Remember, these guys are still part of the power structure that governs where your soul goes after you die, so in the end, "outwitting" them is either meaningless or contradictory.
Again, I'm not asking for an explanation. The writers are clearly not thinking that far along, or else they just don't believe in the afterlife anyway.
Conversely, with "Devil", if M. Night or whoever really gets the credit for the story will resolve it in a different way, I'd say that's refreshing.