[voice]
Sometimes ghosts aren't able to move on and are stuck in the world for some reason or another. Usually it's until they've completed something that was very important to them, but others are because of strong emotions such as sorrow or anger in which case usually someone has to release them before they can move on.
But Acumen arrested a ghost? Well guess that sometimes they can cause a lot of trouble and hurt people, but I wouldn't think you could arrest one.