[spoiler]Is spoiler tag planned? I think it’s important feature which will be really needed in future.[/spoiler]
Maybe in about a year and a half it’ll be more of a thing.
What’s more concerning are spoilers in topic titles, I think.
Unfortunately, these can only be fixed by mods.
But when you read, for example, the abstract title “Chapter 3 - Mission ‘Unforeseen Consequence’”, you can expect that there’s a description of what to do in there because it seems to be a thread aiming at helping players who’re stuck and can’t advance. So, in such case, it would be you own fault if you look in this topic and then get upset about details someone gave away. Same goes for “What are you’re thoughts about the ending?”-kind of threads.
For the rest of the cases, where people deviate from the topic and write about unrelated stuff which reveals important plot points and such, of course, this has to be prevented. 
I suppose you could put it in the title aswell, per regulation or something.
So something like this : "[Spoiler]Chapter 3 - Mission Unforeseen Consequence"
Or : “[Tutorial] Chapter 1 - A new new new Beginning”
Just a thought.