The solution sounds simple. Force non-locals to buy a Swiss-like vignette to drive on those roads. And, make it damn expensive, like 1000 Euros. You can probably put a police checkpoint around a corner about 500 meters after the sign and fine people who are missing the vignette something like 2000 Euros. Finally, use social media and obnoxious free media (City AM and friends) to tell everyone about it. It should take care of itself in about 3 months.