They are dead people who don't know they are dead. A presence is very palpably sensed or seen. There is a feeling that someone is trying to get in touch. In fact, the ghost is bewildered and frustrated because no one can see or hear him. Persons who die while unconscious or insane are also frequently not aware that they have died.
Insane spirits are particularly difficult to help.
Explain gently that they are dead and they need to move on and they will be much happier moving on. Ask them to look around for a light. It may just be a spark of light at first, but it will get larger and larger until the spirit can see a loved one waiting for them in the light. These are spirits of persons who have chosen to stay with a person or place as its guardian spirit. Or perhaps a lonely person likes the ghost's company.