Suppose that you are an editor, reasonably well-informed and conscientious. You deal daily with writers, perhaps also with colleagues who are fellow editors. Part of your triage is to identify which class of colleague you are dealing with in a given situation, so as to respond appropriately.
The Uninformed: Many, if not most, writers write intuitively, "by ear," or by unconscious imitation of the forms they encounter. Since no one ever taught them formal grammar, they get lost in the thicket when there are technicalities and come to you. They come to you because you have been identified as someone who can determine when to use who and when to use whom.
This is the easiest class to deal with. They trust you to have an answer to their questions and accept your judgment without cavil. Even with gratitude. Cherish them.
The Misinformed: These writers did receive instruction. Unfortunately, it was incompetent. They remember not to end sentences with prepositions and shrink from that wicked work of the devil, the passive voice, which they understand to be any clause containing a form of to be. They have uncritically adopted all the bogus ukases of editors who are deader than hot type.
A little patience here will go a long way. Explain gently how they have been led astray and what the skinny is. Marshal your evidence, your citations from Garner on Usage and MWDEU.* Generously offer to forward links from You Don't Say and other unimpeachable sources.
When they go away, they will probably revert to previous practices almost immediately, but the next time it will be easier to reason with them.
The Stubborn: I'm thinking here of the reporter who once said to me, "It's not a cliche when I use it."** By all means employ patience and sweet reason, but it will be an uphill battle. Few certainties are more firmly held than those of someone who is manifestly and provably in the wrong.
Your best hope with this crowd is to wear them down. If that fails, flex your authority. If that fails, go to a higher authority. If there is no court of appeal, settle. You do not want to turn this bunch into adversaries ready to challenge you over every jot and tittle.
The Irredeemable: The prima donna, the star reporter beloved of the bosses, the prize winner who has coasted into semi-retirement at full pay, and all the other monstres sacres.
Pull your forelock in your best crave-a-boon-guv'nor manner, and if you are sufficiently deferential, they may condescend to grant you a crumb. It should be superfluous to say that direct challenge is usually futile. Editing this lot is like developing arthritis; learn to live with it.
The accomplished: You may be fortunate enough to deal with writers who are knowledgeable and competent, who will in fact catch you in some misreading or blunder.
When this happens, accept correction with good grace, apologize, fix what was wrong, and go about your business with a becoming humility.
*Merriam-Webster's Dictionary of English Usage. Please try to keep up.
**Verbatim. I'm not making this up, you know.