Poltergeists are demons. They are not the spirits of the dead. I say this with over 20 years of dealing with them personally. I was a psychic, sorcerer, and occultist for 30 years before I became a Christian. I've astral projected more times than I can remember, and have seen demons, Angels God and Jesus. Just saying.., I kind of know what I'm talking about here, but don't have all the answers.
Scripture says the dead go to sheol ie. "the pit." Sheol in my experience, is actually the upper levels of hell, or what some call the astral plane. The further you are from God in evil/sin, such as hate, unforgiveness, actually enjoying evil doing, murderous thoughts, etc. The deeper you go. This is automatic as God doesn't put up with evil. It happens in the dream world, which is mostly unconscious human astral projection, and for deliberate or accidental out of body experience, and at the time of your death. In the mean time, demons run around the earth tempting man. 95% of what you hear in your head that is negative is not coming from you, but is being telepathically projected at you by these tempters, or they are lies you repeat to yourself that demons have talked you into. ("fiery darts") They can even be family line curses/lies your parents or ancestors believed and have passed down to you. Demons and hell are organized, and work in mobs. So, if you have a poltergeist, odds are there are many demons hanging around supporting the stronger one moving things or knocking on walls, etc. I do a lot of music engineering, and I've actually recorded them talking amoungst themselves about me. They are attracted to psychological weakness, and people with God given gifts, like The Word Of Knowledge (just knowing things about people that are true), Prophetic Gifts, really saintly type people, to name a few. They are also equal opportunist lives destroyers. They don't know Gods plans for people, but they are good guessers. If you're a good person having Angelic or Heavenly communications, odds are you're going to be attacked. In my case I have the gift of knowledge, and used it as a psychic for years, but wasn't attacked until I started using it for God. Poltergeist activity is demonic harassment. The point is to keep you in fear, and in a place of feeling powerless, because demons know that if you have any of God's given gifts you are a serious threat to their kingdom on earth. That's why you'll often see this type of activity around children that have "gifts" even before they know they have them. Occultist/satanic practices like tarot reading and ouija boards are a great way to draw them to you. Demons lie about everything. They hate God, and only want to destroy his works. Unfortunately humans are caught in this war which they can't even see for the most part. They love to pretend to be the spirits of the dead. It keeps people guessing, and allows further and further deception. There's really only 3 ways to get rid of them. Prayer, Exorcism, and Fasting. The stronger they are, and the stronger their hold on you or their legal rights you've given them to invade, plus your repentance time God requires, the further you have to go up that list to get rid of them. Hope this provides some insight for you. God Bless.