The last time they encountered a case like that in 1.08 – Bugs, they actually didn’t break the curse on the land, and a large ghost attack like that is much like a curse. They would have to find signs of the ancient burial ground to get it protected by the state (occasionally works), or find a way to put the spirits to rest if that didn’t pan out. Hoodoo is a better option than a salt and burn, so they’d need to find someone well-versed in the belief and willing to help.