Try soltakss's method below...
It's magic, innit?
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OK, so what you do is use tags, specifically the spoiler tag. Within square brackets [] you put the word spoiler and then in another set of square brackets you put /spoiler.
So, misspelling so as not to have a spoiler, you put [spoileroo]This is a spoileroo[/spoileroo], but using spoiler instead of spoileroo.
There may well be a tag that means "Treat anything inside as text, in which case, I could have used that instead, but not sure what it is. In HTML Code does the trick and that seems to work, so ...
[spoiler]This is a spoileroo[/spoiler]