The EU is Russia's largest trade partner and they are refusing entry of Russian ships and trucks. That is an effective partial blockade.
As for doing it in the high seas, just give it time. I think even the US knows doing it right now would break the world economy. And the US oil price, as their own oil still is traded worldwide, would go to the moon. I think you will start seeing them stop Russian merchant vessels with dry non energy cargo on their "inspections" first.
The whole reason China started their naval buildup was because of an incident where the US blocked transit of a Chinese merchant vessel in the Persian Gulf. I think they were carrying paint to Qatar or something like that. The US claimed they were carrying weapons for Iraq. The US stopped the ship, I think in the Emirates, the crew was impounded and they spent ages inspecting the cargo to find it was exactly as what they had in the manifesto. But you know what, the US never made an official apology. Because "they had reliable intel". Right.