I would say that a lot more signage, like before you even enter the park and at every ride. It could be clearly posted in the map, like they do with fast pass. And after all, every ride has some kind of video. They should have the actors instruct us that way as well.
I really don't think that anything will stop them. Especially more signage.
I have seen people get on M:S KNOWING they are motion sickness prone or claustrophobic who freak out when they get on, despite the fact that they tell you 8,000,000,000 times before you board.
And that is people's health! Tell them not to use their cameras, they will anyway.