Ghosts do exist.  The bigger question is what are they. 

1. One source of ghosts is demons.
   1. They gather around someone is involved in occult practices.  They gain power from these humans.
   2. They gather round someone who is demonized, seeing if the internal demon will give them some power.
   3. They gather in places where witches have sent them.
   4. They gather around people where witches have sent them.
2. another source of ghosts is humans
   1. Shamans can send out their soul to visit places, manifest to people and interact with things.  They do this with demonic augmentation.
   2. Humans who have died suddenly and unrequited can seem to stick around rather than immediately going to Heaven or Sheol.

These demons can stick around for centuries until a Christian happens by and commands them to leave.  That's the only way they will go.  With human empowerment, they can move things, pound walls and make sounds.

 As for the mischievous Shamans, I bind the demons on them to silence & impotence.  They have a wild ride back to their bodies without any of their demon-augmented powers!

There seems to be human souls staying around familiar people or properties.  I find they can't be cast out because they are as human as I.  Instead, I point to heaven, where Jesus is on the throne, and tell them to come to Jesus.  They take my directions and are gone.