This may be a weird way of looking at it, but I don't think you can. Any sort of spiritual realm may be a sort of collective unconscious that becomes visible to certain people at certain times. A lot of "hallucinations" might be "real" in a non-traditional sense of the term. Obviously, hallucinations cannot harm someone physically (psychologically may be a different story), but neither can ghosts. Never has a ghost physically harmed anybody either. So, maybe ghosts are hallucinations, but some hallucinations are also ghosts