A blockade can be enforced in international waters. A ship that’s heading straight towards territorial waters and doesn’t change course when hailed, can be intercepted before it enters.
If you want to actually learn about this, start reading here.
The International Law Association’s Helsinki Principles on the Law of Maritime Neutrality (1998) regulate the same question in Paragraph 5.1.2, which largely overlaps with the provisions of the San Remo Manual. Subparagraph 3 allows attacking of neutral ships that ‘are believed on reasonable grounds to be carrying contraband or breaching a blockade, and after prior warning they intentionally and clearly refuse to stop, or intentionally and clearly resist visit, search, capture or diversion’.
The San Remo Manual and the Helsinki Principles both contain provisions that permit the use of force against neutral ships that are ‘believed on reasonable grounds to be … breaching a blockade’. Blockades are ‘a belligerent operation to prevent vessels and/or aircraft of all nations … from entering or exiting specified ports, airports, or coastal areas belonging to, occupied by, or under the control of an enemy nation’.Footnote 18 Blockades are indiscriminate, in that they must be ‘enforced against every vessel of every nation’
Are nations allowed to set up blockades outside of their EEZ?
A blockade can be enforced in international waters. A ship that’s heading straight towards territorial waters and doesn’t change course when hailed, can be intercepted before it enters.
If you want to actually learn about this, start reading here.
https://www.cambridge.org/core/journals/leiden-journal-of-international-law/article/use-of-force-against-neutral-ships-outside-territorial-waters/56CF204865927A59302B21DCB66D9428
EEZ is not the same as territorial waters.