You could be confused with a normal spirit and a demon. Im not sure if 'demons' are just a fantasy thing, but they are very similar to a inhuman; an entity that was never alive and is there for the purpose of mental manipulation and possession. They are extremely dangerous but can only come if invited. There isn't any easy way of getting rid of them. No seance, ritual or crusifix could stop them. If this sounds like your case, you possibly have a demon. You can ask it to leave in your head, if it won't budge, the other only way is to ignore it.