Well, what I tried to do was completely cleanse a Corruption area. I assumed it's all about the grass, since that seems to be how the Corruption spreads, and other biomes (such as the Underground Jungle) work like that.
So, I bought a few stacks of Purification Powder and Sunflowers, and purified every bit of Corrupted grass I could find.
It didn't completely work, however. Slimes/Zombies now spawn, but so do Eaters of Souls, still — and the music hasn't changed.
Is there something else that has to be done to completely eradicate the Corruption? (I've already smashed every single Shadow Orb; I doubled checked using the World Viewer.)