He was responsible for the launching and subsequent coordination of the Uruguay Round Negotiations and is currently the director of the Trade and Services Division at the World Trade Organization. International trade is a centuries-old activity. The WTO is nothing but a series of legal agreements, freely negotiated by governments, in which they cede some part of their sovereignty over international trade. Of course, they retain their sovereignty in its entirety.
Any government can, if it wants to, ban imports and exports totally as of tomorrow.
No government does so, because the consequences for its own economy and people would be disastrous. But they still retain that freedom, and any member of the WTO could leave it tomorrow. The fact is, they all accept that a world without rules covering the way governments should treat each other, and the extent to which they will interfere with the global trade, will be a dangerous and much poorer world. It's not at all a coincidence that the GATT was created in the aftermath of the world war. People realized how much the disarray over trade in the '30s led to the world war.