I just wanna preface my question by saying that there are !!SPOILERS AHEAD!!. I haven't seen any forum guidelines regarding this sorta thing, but better safe than sorry! 
The nature of how humans read is that we're always
processing text a little bit behind what we're
reading; this means that when we stop reading, there's still a little bit of text that we've already read, but which our brains are still processing. This means that
when dealing with infohazards such as massive spoilers that can cause serious harm to those not inoculated to them, you cannot get away with simply putting a warning at the forefront of a text; you must obfuscate the information in some way. Otherwise, by the time one has finished processing the
warning about the spoilers, they'll already have seen a bit of the spoilers. While the forum software provides collapsible spoiler blocks (insertible via the little yellow-and-black circle in the text editor's toolbar, which admittedly is not the most intuitive interface), i'd be wary about e.g. an incomplete loading of the webpage or interaction with screen readers, so i'd instead use a more robust obfuscation technique like ROT13.