Anyone know of a spell or magic item that could be used to neutralize a ghost from materializing in an area they are haunting?
Basically, anything keeping things from one plane from crossing into another. "Materializing" most often means, for incorporeal beings, coming into the prime material plane from the ethereal plane, the astral plane or the plane of shadow.
So, Antimagic Field or a Dimensional Anchor might both work. I'm not entirely sure about the Anchor bit, though.
Also, Ghostwalk Campaign Setting had a load of spells and techniques for dealing with manifesting/manifested spirits.
Depending on how story-based the game you're playing in/running is, I would say try salt. (Sorry, watching a lot of Supernatural lately.)