In the event of a conflict between the obligations of the Members of the United Nations under the present Charter and their obligations under any other international agreement, their obligations under the present Charter shall prevail.
However, that is not the end of the story. Because the Kadi judgment, it seems to me, reflects another underlying assumption. After all, even if the EU Treaties are so "constitutional" that they have established "a new legal order of international law" (Van Gend & Loos), that still does not change the fact that UN Security Council resolutions are presumptively binding on the EU and its Member States. At the EU level, however, there is a functioning system of judicial review. Kadi again:
285 It follows from all those considerations that the obligations imposed by an international agreement cannot have the effect of prejudicing the constitutional principles of the EC Treaty, which include the principle that all Community acts must respect fundamental rights, that respect constituting a condition of their lawfulness which it is for the Court to review in the framework of the complete system of legal remedies established by the Treaty.Under Dutch law, however, treaties (including the UN Charter) trump the Constitution:
Article 94(Despite the unfortunate translation of the term "wettelijke voorschriften", this provision applies to the Constitution itself as well. This follows from art. 91(3), which establishes an extra-difficult ratification procedure for treaties that conflict with the Constitution.)
Statutory regulations in force within the Kingdom shall not be applicable if such application is in conflict with provisions of treaties that are binding on all persons or of resolutions by international institutions.
So if Seselj brought the Dutch equivalent of a Habeas petition, the court would simply reply that his detention is lawful because it is pursuant to a Security Council resolution (Resolution 808, to be precise). Which leaves only one question: If this Dutch judgement were appealed until all national remedies were exhausted, and followed by an appeal to the ECtHR in Strasbourg, what would the Strasbourg court say? Would they be prepared to impose a damages award in these circumstances?