Not sure if you use them, but the Strange Aeons rules handle ghosts very nicely, IMHO, especially in regards to poltergeists. (The official SA ghost rules are free to download from the Strange Aeons Web site.)
Essentially, ghosts are immune to all physical attacks (except those from "blessed" weapons). However, ghosts may still be defeated by enemies without such weapons. In the case of the poltergeist, opponents' models may search the board for the ghost's mortal remains. Once the remains have been found, a model may spend its turn to purify them and dispel the entity. (For some reason, I have a mental picture of Sam and Dean from Supernatural digging up a grave, the salt and gasoline waiting nearby...)