Demons can possess a person without his consent.
Could you please elaborate (great topic BTW).
Anything other than this would violate the basic but universal "law of free will."
This position is supported by many, many discussions with theologians, priests, and ministers and by writers on the topic of possession and exorcism like Malachi Martin (Hostage to the Devil) and, to a lesser degree, Scott Peck (People of the Lie).
That said, it is possible that something, some exception, has been overlooked.