“Are you actively killing the Grapevine?”
Lopp speaks of this cancerous Grapevine within a company. Do you feed it, or do you kill it? It's easy to feed it, because, as Lopp points out, "Being part of a secret feels powerful."
Without active prevention, the Grapevine can be stronger than any individual. While you can’t kill the Grapevine, you can dubiously stare at it when it shows up on your doorstep and simply ask the person delivering it, “Do you actually believe this nonsense? Do you believe the person who fed you this trash?”