The simple solution is to only allow trading on live events and remove it completely for pre-recorded content. Even with NDA’s there are way too many people who know the outcomes to keep everything a secret.
Update: we continue to investigate insider trading in Survivor markets, but the below still holds true: so far, no traders with large positions seem to have a relationship to the show or to the network. Thousands of traders took early positions consistent with the outcome, likely because of public rumors. And public rumors don't equal insider information.
PS—as a longtime Survivor fan, big congrats to Aubry. Well-deserved win.