Europe has always been known for being governed by the rule of law. If we now start breaking laws and rights, especially regarding property/ownership, this will strongly backfire in the future. This can quickly become a slippery slope towards Willkürsjustiz. It is exactly the same as with the Russian assets held in Belgium at Clearstream. Selling them is a no-no.
There is ample precendent for impounding the assets of hostile nations. The Soviets did it to Germany in WW2, so they cannot really claim that they are opposed to that practice.
The only reason why this seizure of russian money in Belgium might be a bad idea is reciprocity. Russia would of course then try to seize European assets in Russia.
And regarding ships, prize law is still internationally accepted and in effect. Ukraine can offer prize letters to privateers or foreign navies, allowing the seizure of Russian ships. Or they can seize ships themselves. When those ships are then in a Ukrainian or allied harbor, a Ukrainian admirality court then assigns ownership of the vessel and all goods to the ones who brought it up. https://en.wikipedia.org/wiki/Prize_(law)
