How do you 'prebunk' conspiracy theories?

You read the signals from the edges. It's tough but not impossible. It's what good Intelligence is based on. You do this — 1) Systemically e.g. the Finns are less susceptible to fake news because of the school system & universal service 2) By watching for weak signals in forums and other alt media 3) Tell a more compelling story e.g. the film Casablanca (1940) — it takes a village to raise a citizen.

One other thing that can easily be done is not to quote the conspiracy or report on it. Kill it through indifference.