Facebook should put a clause on their terms of service stating that users may not share their password with anyone else without first informing Facebook that they intend to do so. They should create a special form you can fill out for the express purpose of informing Facebook that you intent to share your password with someone. This form should request enough data that Facebook will be able to find that individual's Facebook account.
Then Facebook should use this information to either a) ban all employers reported through this mechanism, or b) set all their privacy to the lowest possible setting without the option of raising it back up.
Fair warning would be set out in the terms of service ("By using this service, you agree to.."), and they could use selective publicity to very loudly announce that you have to report to Facebook if your employer requests your password while being more discreet about the consequences for the employer. Facebook already has a reputation for changing privacy settings and terms of service on its users, so it may as well use this precedent for good.