A rumor can persist when there's a vacuum, especially when it's referring to events that haven't happened yet.
Here's an example: "An asteroid four-miles wide will hit North America on May 3, 2013 -- but NASA is keeping it a secret because they don't want people to panic."
Lies are different. The facts are known.
"The earth is flat" is a lie. It's simply not true. There are thousands of pieces of evidence that the earth is round, including live photos from orbiting satellite.
Flat-earthers don't have a shred of proof. Yet they still believe that the earth is flat, because they want to believe it.
That's the biggest problem with lies. They are easy to disprove, being factually untrue. But that's not the real battle.
The tough thing is for a lot of people -- because of ideology, personal beliefs and other reasons -- don't care about proof and evidence and facts.
They want to believe certain things, whether they are true or not.
That's why defending against lies isn't a matter of simply showing people evidence of the truth.