What can other WTO members do against the American move to block the WTO judge reappointment? Is the WTO effectively frozen as the U.S. blocked the WTO judge reappointment? What can other members do about it? How come a single country can make such an important decision?
First some quick background. The US has been blocking appointments to the Appellate Body (AB), a body of 7 judges of which 3 serve on a given case. There are currently only 3 members, 2 of whom have their terms expire December 10th, which will leave the body non-functional. Judges are nominated by the Dispute Settlement Body (DSB) whose composition is all WTO members. In a dispute between two members, the DSB will establish a panel to adjudicate, and the losing party may appeal to the AB.
Is the WTO effectively frozen as the U.S. blocked the WTO judge reappointment?
Yes, the normal dispute resolution is frozen. Members have the right to appeal, and this will now turn into a so-called "appeal into the void."
How come a single country can make such an important decision?
Per Article 2.4 of the Understanding on rules and procedures governing the settlement of disputes, DSB decisions are by consensus, including appointment of AB judges.
What can other WTO members do against the American move to block the WTO judge reappointment?
Basically, the members can agree in advance to alternative resolution mechanisms. See this paper for an explanation of the available options: Pauwelyn, Joost, WTO Dispute Settlement Post 2019: What to Expect? What Choice to Make? (July 7, 2019). I'm just going to summarize the two options which lead to a binding dispute resolution.
No Appeal Pact: Members agree not to appeal and take the DSB panel report as binding. The main problem with this solution is current WTO dispute practice. In 89% of cases, the panel finds at least one violation, yet 83% of appeals reverses at least one aspect of the panel decision. Appeals are thus very useful for defendants, and so this solution is only attractive to members who are more often claimants than defendants.
Appeal to Arbitration. Article 25 provides for binding arbitration as an alternative to the normal DSB/AB route. The European Union has proposed what basically amounts to a substitute AB. It proposes combining the No Appeal Pact with an appeal to Article 25 arbitration where the arbitrators will be former AB judges (presumably there's enough of them around). Since the paper's publication, Canada and Norway have signed on to this solution.