The Washington Post also reported on the Zarif sanctions, and the article quotes an administration official offering one of the most preposterous justifications for the action:

The administration official said Trump remains ready to speak with Iranian leaders — just not Zarif.

“If we do have an official contact with Iran, we would want to have contact with someone who is a significant decision-maker,” the official said when asked whether sanctioning Iran’s chief diplomat would limit U.S. ability to negotiate with Iran, if negotiations ever take place.

Zarif “would not be the president’s selected point of contact,” said the official, who spoke on the condition of anonymity under rules set by the administration.

The administration’s position is risible nonsense. Heads of governments don’t typically conduct negotiations with each other, and the U.S. would never refuse to deal with the foreign minister of any other government. If you don’t want to negotiate with a government’s fully empowered chief negotiator, you don’t want to negotiate. The JCPOA was negotiated and concluded without ever directly involving Rouhani or Khamenei in the talks. The talks were entrusted to the foreign minister, just as negotiations of this kind always are. That is what chief diplomats are supposed to do. The “significant decision-makers” have said that Zarif is the one responsible for handling these things, so spitting in Zarif’s face and saying that he isn’t important enough to talk to are insults to them, too.

Putting sanctions on Zarif isn’t just an affront to the Iranian government, but also to the Iranian public that identifies him with the cause of engagement and the nuclear deal. Esfandyar Batmanghelidj explained this earlier today:

2. Moreover, for the Iranian public, Zarif and the prospects for diplomacy are one and the same. By sanctioning Zarif, the US is sending a clear signal that it wishes to undermine not only Iran's chosen diplomat, but also its vision of diplomacy. That includes the JCPOA.

— Esfandyar Batmanghelidj (@yarbatman) July 31, 2019

The insult has provoked a predictably display of unity from Iranian officials:

Iranian officials reacted with unified irritation on Thursday to the Trump administration’s decision to sanction Iran’s foreign minister, calling the move petty and provocative — further evidence, they said, of Washington’s insincerity when it talks of peace.

The decision has been roundly condemned in the U.S. as well:

“The Zarif designation is one of the most ridiculous steps I’ve seen this Administration take,” tweeted Richard Nephew, a Brookings Institution fellow who helped oversee the expansion of sanctions against Iran under the Obama administration and served as sanctions adviser to the American team that negotiated the nuclear deal. “It won’t meaningfully affect Zarif’s diplomacy, it probably won’t result in much in terms of asset freeze or complications for him, and will annoy other world leaders.”

The EU said that it would continue to work with Zarif, and leading U.S. allies in Europe have likewise expressed their opposition to the decision:

France said on Thursday voiced concern over a U.S. move to impose sanctions on Iran’s foreign minister and said that along with Britain and Germany it disagreed with the decision, the French Foreign Ministry said.

“We consider that all diplomatic channels should stay open, particularly in a context of high tensions,” the ministry said.

Sanctioning Zarif was just the latest sop to hard-line Iran hawks in a long line of terrible decisions that Trump has made over the last fourteen months. Reneging on the JCPOA made it so that Iran had no reason to want to talk to Trump, and now Trump has made clear that he is repudiating diplomacy with Iran.