In GP, Harry goes on about how ghosts are really hard to harm or affect at all - you have to get them to acknowledge you (at least if you're not in the Nevernever?) and Harry makes ghost dust out of weird, exotic, hard to get ingredients like depleted uranium to fight ghosts with.
But in GS, ghosts seem way more vulnerable. Sir Stu is nearly destroyed by fire, and Harry as a ghost talks about how running water could destroy him.
But if plain old fire could destroy ghosts, why bother with ghost dust? Just use a torch or something...