No single country can block these. The dispute settlement system of the WTO is a central element in providing security and predictability to the multilateral trading system and WTO members commit themselves not to take unilateral action against perceived violations of the trade rules but to seek recourse in the multilateral dispute settlement system and to abide by its rules and findings. The DSB monitors the implementation of the rulings and recommendations, and has the power to authorize retaliation when a country does not comply with a ruling. Before taking any other actions the countries in dispute have to talk to each other to see if they can settle their differences by themselves. If that fails, the two countries can also ask the WTO Director General to mediate or try to help in other way.
Panels are much like tribunals.
Unlike tribunals, however, the panelists are usually chosen in consultation with the countries in dispute. When the two sides are unable to agree on the composition of panelists, the WTO Director General then has the authority to appoint them. This happens rarely. The panel's report is passed to the DSB, which can only reject the report by consensus. Panelists for each case can be chosen from a permanent list of well qualified candidates, or from elsewhere.