Computer security problems aren't that different from "real world" ones: it's a matter of trust. Either you believe that someone will do what you want, or they deceive you. While it's reasonable to have some systemic protections, at some point it is entirely up to the individual.

You might leave someone to watch over your house, and they could decide to trash it; that violates your trust, but it doesn't hurt your neighbor's house. You don't tie up every citizen to make it impossible for anyone to ever trash another house; you simply deal with that one trust violation, and learn from the experience. Also, you should tell others, to minimize how many suffer the same fate.