You can’t stop every rumor, but you can be the place where one dies.

A rumour is not an object, it is a chain, and a chain needs every single link. You cannot control who started it or how far it has already travelled, but the stretch of it that runs through you is entirely yours. When it reaches you and stops, one whole branch of it is gone for good.

The method is boring and it works. Do not pass it on, do not ask a follow-up question to get more of it, and do not repeat it in the shape of a question. If you want to do more, say plainly that you have no idea whether it is true.