When it comes to my codebase, the main result of unreadable code is that i would refuse to merge a coworkers change into the main branch and send them back to their desks. With a long todo list on what to fix in their code.

It's important to understand that i don't shout. Looking sad and slightly shaking my head in disappointment seems to be way more effective when it comes to improving code quality, especially when they come back with still unreadable code. This seldomly fails, my coworkers know the next step up is the dreaded re-run of my one-hour "how to write maintainable code" eye-to-eye lecture while they sit at my desk, watching me refactor their code, commenting on every single mistake and showing how its done correctly.

It sounds harsh, but it isn't. It's never "you are too stupid to do this right". It's always "You need to learn how to do this to my coding standards. We pay you for it, so take the time and do it. And this is how it's done: ..." (And yes, if a coworker has a good argument on why his way of doing something is the better way, i am not above spending a weekend or two to adapt my whole codebase to the new way of doing things).