There is the idea of segregating anything spoilery into its own board, which I've seen on other roguelike forums, but I think that might be a bit limiting. Spoilers can logically find their way into discussions on any number of boards, like Stories, Strategies, Competitions, or even Ideas.
I think you can indicate spoiler content via thread tags, if you start the thread and the whole idea of it includes spoilers (and should obviously avoid describing the exact spoiler in the title itself), though of course the safest way beyond that is to use the [spoiler]
BBCode tag in your post itself.
Or maybe add a dedicated spoilers board anyway, to mostly keep it concentrated there (and allow for any content in post titles, which makes discussion easier to find and follow!), but then if a discussion elsewhere wants to mention a spoiler within its content, it can still do so with the BBCode.
I guess the question is: Does anyone want a Spoilers board?