Well, according to the law of Murphy even if you make a rule "no spoiler allowed" it's still fairly certain that someone new to the forum (and without the patience to read the rules) from time to time will post some spoiler anyway.
I would stick to the idea of creating a forum section dedicated to spoilers (preferibly put on top so that anyone can see it), or to add a spoiler button (recognizable with a biohazard or even better a yellow/black nuclear sign) that, as a result, make appear a text area with the word "Warning!", "Spoiler ahead!", "Click at your own risk" or something like that.
