Yes. If I was drafting that person’s will, I would have the individual recite the knowledge he does have regarding his family (to prove that he is of sound mind). He would state that even if one of those individuals comes forward, he does not acknowledge them as a family member for the purposes of this will.
After that, the individual declares that he disinherents all of them, living or dead, children or not, named or unnamed, known or unknown, nor does he wish that any state law of intestate succession apply to those people.
He leaves everything to the local pet shelter.
Of course, just to keep in mind, the only person in most (all?) states a person cannot completely disinherit is a spouse. If you have a will that says “I leave everything to Shadyside Pet Shelter” then everyone else can go fuck off so long as you are of sound mind. If long lost nephew comes along, then the will controls anyways.
That is why the recitation of family history is a good thing to include in a will. It shows a judge that the person was of sufficient mind to articulate to his attorney his immediate family members, but is not at all necessary for a valid will.
I don’t think there is any case law, but I would be shocked if any judge or any medical provider would look at a clear directive on a POA that says, in effect, “I don’t want Bob making any medical decisions for me” and then allow Bob to make medical decisions for that person.
ETA: On some issues, the lack of case law supports your position because you can argue that the issue is so blooming fucking obvious that nobody decided to waste money by arguing the position in court, or even if a couple did, they certainly didn’t appeal the ass chewing they got from the judge and it does not reach a published appellate decision.
What about a document where you explicitly list out who you want to be considered as a relative and their hierarchy for all cases where you have not explicitly already stated it? For example, maybe you have a will, PoA, and MPoA, but you still want to ensure that only a certain group of people are considered your relatives in any situation you didn’t already consider. So maybe something like:
Something like this would also allow you to change the default relative order if you wanted it different than what the state specifies.