Yes, cities should shame Johns, because shaming them would deter future crime. It is too often the women that are penalized for these crimes, but the men are not looked down on as much. Public shame would make men think twice before committing crimes of this nature, knowing that they could end up on a billboard as well.
Arrested Johns are not really the equivalent of registered sex offenders or child molesters. I think that their actions have reaped their own consequences and it isn't for authorities to shame them further by broadcasting what is considered deviant or unacceptable practices or behavior. Their own punishments and consciences are enough.