Societies turn people into ghosts by excluding them from their normal activities. A person who might be sociable enough makes a small mistake and the society decides to seclude them from rest of the world. This seclusion leads to loneliness which turns them into ghosts. The secluded person might not like to meet others any more and would remain unheard of. This would make him a ghost, i.e., nobody would know what happens to him and he would remain unheard of. People are likely to create stories about these kind of people. This can also happen in a group or in a large scale.Racism is a large scale way of societies of turning people into ghosts. in the context of "to kill a mockingbird" Boo Radley is turned into a ghost because of the fear that is instilled into the minds of people through useless rumors and gossip.