Have you tried smudging? That can clear negative energy. I usually do it when i move into a house - not sure if it works outside the house, but you could circle the house.

I've googled stuff on smudging and ghosts:
How To Get Rid Of Ghosts
Smudge a House to Get Rid of Ghosts - Associated Content - associatedcontent.com
Ghosts 101: Haunting First Aid, How to Reduce the Scariness/Malevolence
Smudging/ Burning Sage and GOOD spirits - Ghosts - Supernatural Message Boards
smudging ? Spiritual Jewelry & Personal Development Tools

They seem to indicate the ghost needs to move on before smudging, or they just come back.

btw - are you 100% sure it's not some local weirdo hanging around? Don't want to freak you out, just concerned.