I tried a bunch of blocks in creative mode. Most explosion-resistant blocks survive the initial Wither explosion when it spawns. However, everything I tried got destroyed by its projectiles. (Obsidian, Basalt, Reinforced Glass/Stone, Iridium Reinforced Stone, Hardened Glass, Mazestone, Xychoridite Shields, Warded Stone...) It seems that the only feasible way to contain a Wither are forcefields. Even with that I could see it break the forcefield blocks with its projectiles, but the forcefields got instantly replaced.
(Mindcrack pack v6 MC 1.4.6)