He cites two legal documents that ties Brussels hands:
1. Association Agreement with Ukraine, Article 276 - picrel #1
For this specific situation: Ukraine can't block the flow of Russian oil.
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=LEGISSUM:280101_1
https://eur-lex.europa.eu/eli/agree_internation/2014/295/oj/eng
https://publications.europa.eu/resource/cellar/4589a50c-e6e3-11e3-8cd4-01aa75ed71a1.0006.03/DOC_1
2. Council Regulation (EU) 2022/879 of 3 June 2022
Croatia must allow the transfer Russian "seaborne crude oil" via the Adria/Janaf pipeline to us.
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32022R0879

What we can do, beyond petitioning Brussels:
- stop the diesel export towards Ukraine, this is about the 10% of Ukraine's diesel usage
- block €90 billion relief
- 40% of gas and electricity import of Ukraine comes from Hungary, 3 billion cubic meter's of gas (supplies about 3 million households), and 2 TWh (terawatt-hour) electricity (supplies about 1 million households)

We stopped the diesel export. What Ukraine buys from us is refined from the Russian crude.
We block the €90 billion until either Croatia lets the Russian oil in, or Ukraine restarts the transport.