So don't post it online to begin with. There are things that simply shouldn't be posted online under any circumstances.
That doesn't only mean "don't sext", but also, for example, if you're a corporation or a public entity, don't have personal information stored in a server that's online unless it's completely necessary, etc.
By having something connected to the internet, someone can make a copy, and once the copy is made, there's no going back.