If you're going to say something about a movie, show, etc that reveals plot elements or secrets and the like, PLEASE be considerate and use the lovely, handy-dandy [SPOILER] [/SPOILER] tags. Anything you post in them will be hidden from view unless someone hits the button to show it.
If you are starting a thread that contains spoilers, be sure to put [SPOILERS] or (contains spoilers), or something else to that effect IN the subject line, so people know in advance not to bother if they don't want the story revealed.
Just be considerate and remember the tags. It's not really a hard and fast rule, just some advice on how to use the tools available.
For the record though, if it looks like someone is deliberately posting in threads *just* to blurt out spoilers to be a dick, they will probably get slapped with a trolling infraction.