What does wheeler mean when he said They thronged up the sponge

He basically means they have given up, or acknowledged their defeat. Thronged up the sponge has the same meaningful value that "throwing in the towel" has but throwing in the towel is just more common.

panem is a communist totalitarian dictatorship where all the resources and how they are allocated belongs to the capitol. the districts exist only to provide goods and services for the capital and most of the citizens live in poverty. it is a police state where the "peacekeepers" use force and threats to keep the people under control. dissent is outlawed and those who disobey the law are executed, flogged, or turned into avoxes. the games are another means in which the ruling elite in the capitol exert their power over the districts. when reading the books i drew parallels between panem and the soviet union and north korea. in no way is panem capitalist. the people in the capitol are wealthy but the majority of them are involved in the government somehow. it was the same way in the former soviet union where bigwigs in the communist party and high ranking government officials were given access to goods, services, medical treatments not available to the general public. if you think panem has a capitalist economy you need to brush up on your economics.