"Atticus said no,it wasn't that sort of thing,that there were other ways of making people in to ghosts."What did Atticus mean by tyhi sremark?

In this quote, Atticus is alluding to the fact that although Boo is well and alive, he's been incarcerated by his family and locked up to the point where he simply no longers exists..... except as a source of gossip and wonder. There are many ways for a person to disappear...... to be kept out of sight, but Boo's disappearance from the public eye has been imposed rather than chosen. Because he's never seen..... Boo is seen as a ghost.