Saying “that’s not true” about a rumor, even quietly, slows it down.

Rumours move on momentum, and momentum is more fragile than it looks. One person saying it is wrong, with no heat and no audience, forces everybody listening to carry two versions instead of one, and a story that needs explaining travels far more slowly than a story that does not. Winning the argument is not required. Slowing it down is enough.

Use the shortest version you have, in a flat voice, with no follow-up: that is not right. Say it in a small group rather than waiting for a moment that never arrives. If you know the person being discussed, it is worth telling them what is going round.