There is no spoiler tag in basic BB Code. It's something added in. Since I'm using the free version of the forum, I can't add it in myself. It's been suggested to the host to add it in, though. If my finances become stable enough, I'll consider paying out for the advanced features, allowing me to do that. For now, you can use workarounds like coloring the text to the default background, so it can't be easily seen.