As other member said, spoiler tag isn't exist. But looking at your signature, i would suggest you to put NSFW warning (with bigger font size, bold or red/blue color) at top of your thread instead.
I remember seeing someone with a live cam porn signature in the past. so I thought an NSFW warning will only be needed to be put if the NSFW content is on an avatar, on a post, or thread. that being said, the OP's thread about the live cam site should have an NSFW on the Title.