Are there ghosts walking the earth?
No. It is given to us "once to die, then the judgment" (Heb. 9:27). That judgment determines where we go for the rest of eternity. Earth is like a layover point during a flight. Once you leave, you can't come back. Demons, which are fallen angels, do walk the earth, and they are likely the ones who are masquerading as ghosts.
In the story of the rich man and Lazarus, Jesus said there is a barrier that blocks the dead from leaving hell. It is therefore impossible for someone who was unsaved in life to be walking the earth in death.
"And beside all this, between us and you there is a great gulf fixed: so that they which would pass from hence to you cannot; neither can they pass to us, that would come from thence" (Luke 16:23).