Improvising will have to do. There should be a source of the curse. It may only be my world that operates like this, but my experience as someone who has hunted Witches has taught me this. If you can find the central point and destroy it, the curse will be destroyed.

That being said, it is likely that from that point on, one of the other ghosts will become a new central point. You may need to be prepared for a long, hard battle. You may even have to fight all of us to end this.