Presumably the police don’t enter the embassy in question, haul the diplomat out, and put him on a plane home. So what is the actual procedure? Is any actual force used, or does everything rely on the diplomats following diplomatic protocol and leaving voluntarily once their visas are revoked?
It relies on diplomats following protocol, for the most part. When a diplomat is expelled, their diplomatic credentials are revoked and they are usually given a few days to get out. After that deadline, they become illegal aliens and are subject to deportation. In practice, no country wants their diplomats hauled before an immigration court, so they GTFO when told to do so.
Apart from anything else, it will at some stage involve a reciprocal removal of diplomats, so it’s in the interests of both countries not to make it a violent exercise.
See the first few paras of this:
And at that point, they would, in the language of the 14th Amendment, become “subject to the jurisdiction” of the United States, or at least the U.S. would be asserting jurisdiction over such a recalcitrant diplomat to the extent of bringing him before an immigration court, since that’s almost the literal meaning of ‘jurisdiction.’
Am I right about that?
If a foreign diplomat refused to go, what would they be wanting to stay for? They couldn’t be acting as a diplomat, since one or other country would by definition refusing the usual diplomatic channels of communication (and if one government does, the other can hardly say they don’t accept it, nor is it an individual diplomat’s decision whether or not to do so), so they wouldn’t be of any use to their employing goverrnment; the only reason for them to try and stay would be in effect to switch sides and ask for political asylum.
Jurisdiction means lots of things. The text of the constitution provides for original jurisdiction in the Supreme Court in “Cases affecting Ambassadors, other public Ministers and Consuls,” after all. I suspect there’s almost no case law regarding forcible deportation after revocation of a diplomatic visa.
A Venezuelan deputy consul stayed on in the US this past summer after being declared persona non grata. He was attempting to apply for a green card. I have not been able to find any updates after the initial story.
Article 9 of the Vienna Convention on Diplomatic Relations foresees a situation in which a diplomat is declared persona non grata but does not leave the country:
That means that diplomatic immunity would no longer apply.
The question I’m trying to get at is, if someone in the United States is not subject to the jurisdiction of the United States, but has become a problem that won’t voluntarily go away, what’s the toolkit for dealing with that person?
Do they at some point become subject to its jurisdiction, or are there things that can be done (e.g. involuntary removal) that aren’t considered an assertion of jurisdiction?
This seems to be a relevant question because the President has declared that several million persons currently within the U.S. aren’t subject to its jurisdiction.
Now if a court has no jurisdiction over you, it cannot try a case in which you are the defendant. It cannot hand down a decision depriving you of life, liberty, or property. So it would seem that an assertion of no U.S. jurisdiction over a class of people would indicate that at least the domestic laws of the U.S. do not apply to them.
But take the for-instance of a diplomat who decided he didn’t want to leave once his diplomatic credentials are revoked. Does the revocation of those credentials mean that staying puts him under U.S. jurisdiction? If not, would it somehow not constitute an assertion of jurisdiction for the Feds to nab him and show him to the border?
ISTM that the answers to these questions, if there are answers, have bearing on the situation that Trump asserts that undocumented persons are in. There are things I’d like to say about the legal implications of these persons not being subject to U.S. jurisdiction, but I also don’t want to be talking out of my ass when I do. 
Took my time with my previous post, and didn’t see this one until after I’d posted it.
At that point, such a person, no longer having diplomatic status and being persona non grata, would fall under U.S. jurisdiction. Presumably a court would be asked to rule that the Feds could show him to the border (or put him on a plane back to his home country), and that ruling wouldn’t be enforceable if the court didn’t have jurisdiction.
I don’t think there is one - because there doesn’t seem to be any need for one for the two groups of people who can definitely be in the US and not subject to the jurisdiction of the US - diplomats and their families and soldiers of an invading/occupying force. As a practical matter, there wouldn’t be anything we could do about the soldiers so lets stick with the diplomats. Diplomats are not subject to the jurisdiction of the US while they are diplomats - but once the US has refused to recognize the person as a diplomat or the home country has waived his or her immunity* the person is then subject to US jurisdiction.
- the person him/herself has no choice in the matter.
So basically:
In the U.S. + not recognized as a diplomat => subject to U.S. jurisdiction.
So contrapositively, in the U.S. + not subject to U.S. jurisdiction => foreign diplomat.
Guess we have a few million new diplomats in the U.S. 
It’s hard to be a foreign diplomat if you are not representing any body (like a sovereign state) and have no diplomatic credentials.
Options seem to be: 1) They never leave the embassy, and are basically under house arrest like Julian Assange. 2) They leave the embassy, are arrested, and are deported.
The procedure goes:
Diplomat gets appointed by home country.
Diplomat goes to the host country and presents credentials.
Credentials are accepted by the host country.
Diplomat now has diplomatic immunity.
Diplomat commits an offense of some kind.
Diplomat is declared persona non grata by the host country
Their diplomatic credentials are revoked.
The diplomat now does not have diplomatic immunity, and are no longer a diplomat.
But this isn’t retroactive, they still have immunity for their previous actions.
They no longer have permission to stay in the host country.
The ex-diplomat boards a plane and goes home.
But what happens if they refuse to go home?
Then the home country orders them to go home.
What happens if they refuse to obey the order to come home?
If they ever leave the embassy, then since they are no longer covered by diplomatic immunity, they could be arrested and deported by the host country just like any other person who does not have the legal right to be in the host country. They still have immunity for actions taken while they were still covered by diplomatic immunity.
If they don’t leave the embassy, then they’re subject to the home country’s authority. They could be arrested, handcuffed, gagged, and stuffed into a diplomatic pouch. Or strangled and have their fingers cut off. Or just not given food from the embassy kitchen.
Or the home country could waive diplomatic immunity for the person, and hand them over to the host country. Diplomatic immunity belongs to the home country, not the diplomat. The diplomat only has it because he represents the home country, and it is the home country that holds the privilege, not the diplomat.
Incorrect contra positive. The negation of and is or. So the contra positive of “If you are in the US and not recognized as a diplomat, then you are subject to U.S. jurisdiction” is “If you are not subject to U.S. jurisdiction, then either you are not recognized as a diplomat or you are not in the U.S.”.
For that matter, if the recalcitrant diplomat refuses to leave the embassy, the host country could go in and arrest him. It’s a popular misconception that foreign embassies are foreign soil, but they aren’t. They are protected by the aforementioned Vienna Convention and traditional practices of international law, as well as practical considerations of reciprocity, so it’s not the same as just staying in a hotel, say, but the protections afforded are not absolute.
When the US entered WWII the federal government placed German, Japanese, & Italian diplomats under house arrest at a resort hotel until they could be exchanged for American diplomats via a neutral country (IIRC Portugal) while the Swiss took custody of the actual embassy buildings.