Well, D&D didn't invent ghosts, and they have been combatted in works of fiction without resorting to impaling them on magic swords.
I'm trying to recall some movie I saw when I was a kid. I believe it was called "Spectre" and it starred Robert Culp. He's an occult investigator who battles spiritual evil. The only scene I recall distinctly is one where he explains that while spirits can ignore our physical laws, certain words or symbols can stop them as effectively as brick walls can imprison a human. Maybe Knowledge Relgion or Planes will be a useful tool?
I always liked the idea of heroes actually having to figure out a monster's weakness in order to defeat it, rather than simply expect everything up to and including a tarasque to succumb to brute force. Then again, I guess with the emphasis on "ass-kicking" I've heard so far, I suspect IL will not be too different from vanilla D&D in this respect.