These emojis of warning signs intend to prohibit or restrict something. Copy them to your social media if you don't want your page to be littered or smoked like a joint, if you don't like it when people trespass the territory of your posts, or when you need to tell a stranger to keep their bicycle off your news feed.
Apart from that, these emojis will perfectly fit radioactive, toxic, sickening, and socially dangerous pages, accounts and websites.
Of course, you can also mark your content with the sign 「18+」. Do that if you want to attract more schoolers — they are drawn to it like moths to a flame.