What an interesting question. I would argue that the ghosts that exist in this story are the ghosts of tradition. The community Emily is a part of is resistant to change, as much of the South was following the Civil War. As most humans, they cling to what they have already known. They cling to laws and traditions that no longer exist. For example, Emily clings to the tradition that allowed her not to pay taxes. That tradition died with Colonel Sartoris, but she clings to the ghost of it nonetheless. And the new town leaders, the younger set, are so influenced by these ghosts of tradition that they do not enforce their own laws. Clearly, the ghosts exist and are powerful.