Who watches the watchmen?

Before anyone gets the wrong idea, no, I'm not talking about the movie/show The Watchmen. I'm referring to the ancient philosophical question "quis custodiet ipsos custodes" or "who watches the watchmen". Go read up on that elsewhere.

For those of you who don't know and need a summary here, it's a question often posed in reference to the fact that the person or people in charge of making sure the rules are honored have nothing preventing them from disobeying the rules. There's never anything preventing the person guarding your treasure from stealing some of the treasure, for example.

What's the best remedy to this that you can think of?