I refer to such "Warners" as having the "Good Scout" complex. They really need to "Help" people by warning them beforehand.
I will tell a person not to do this a few times but still the best way to handle this is to have alternate scare plans in effect just for these "Scouts".
Possibly just having an extra scary actor roaming around could ruin the Scout's plans too.
The newbie seeing the "Scout" scream and jump would probably diminish the value of his information and predictions in the mind of the ones he was trying to "Help".
When the "Scout" is giving pre-scare information to a bunch of your customers he doesn't even know, a stern warning should be given because very possibly those ticket-buying strangers WANT the full effects-Scares they paid for?
How unpoular would someone be sitting in a movie loudly telling everyone what was going to happen next up on the screen?