Real people in the real world understand that rules don’t simply cease to exist because there’s no technical means of guaranteeing their obedience. You simply ask people to follow them, and to affirm that they’re following them whether explicitly or implicitly, and then mete out severe social consequences for being a filthy fucking liar.
The hope here is the same hope as most laws: that lies eventually catch up to people. That truth comes to light. But sure, in the meanwhile, there are always dishonest people around trying to flout rules to varying degrees of success. Some are caught right away, some live their entire lives without it catching up to them. That doesn't make the rule unenforceable, that just highlights the limits of rules: it requires evidence that can be hard to come by.