Have you thought of this as an idea?
I wanted to write a comment about a TV show to those in a thread that have seen it but other people have only just started watching. Perhaps in the same way you can use * to bold things, there could be a feature where you highlight the text you want hidden and then click a button and something like [SPOILER] wraps around the text and then those that want to click it can and those that don't, don't?