For the observant among you, there's now a "Spoiler" button next to the strangely labelled "Big Input" button. For the love of all that's holy don't click it. It doesn't do what it's supposed to do.
What you need to do instead is type [spoiler] [ spoiler ] at the start of the text you want to hide and [/ spoiler] [/spoiler] afterwards. Only without the spaces between the square brackets and the word "spoiler"