Yes, it is possible to be controlled by demonic forces. It has been happening for years and years. Many Saints have been tormented and taunted by demonic forces. I believe that the closer you become to God, the more harsh these forces are. There are many people in the world that perform rituals just for that purpose...and then there are many others who "see" with eyes and know demonic forces exist and recognize that spiritual warfare is constantly being played out on this Earth.
Yes, one of your responses speaks about the Bible and that is true. For one to understand God and the Bible, you must acknowledge that God is in the "spirit world," with much supernatural power also. God is always more powerful and with that thought, only God allows these forces to be at work. There is a reason for God to allow evil to taunt/seduce and possess. He is sorting out his "team" and wants to know where your allegiance is...for GOOD or for EVIL. It is as simple as that!
Where does your allegiance stand? With good or with evil? There is no sitting on the fence with this question.
I will add that one can not be possessed by demonic forces if protection is around them and they know God, goodness, Many possessions have happened because someone has invited demonic forces in-willingly.
Most of these possessions, I believe can be invited in through addictions, drugs/alcohol, the occult. Yes, many have been made to go insane, mental illness due to demonic possession. Not the other way around. Nothing to joke about. BabaSixto has it going on. I believe he has experienced much more than he states and is an 'igniter,' in some sense of the word.
In this world there is always a constant warfare being played out around us-seen and unseen. Soon the heat will be revved up and we will be forced to chose our allegiance. Which will yours be too?