Terraria – How does corruption spread

I believe a Hallowed Biome will prevent them from spreading. These can be created by planting Hallowed Seeds, which are gained by smashing a Demon Altar with a PwnHammer.

Edit: I've discovered I was mistaken about how to get Hallowed Seeds. You don't directly get them from the Demon Altar, but instead buy them from the Dryad after smashing the altar.

With the Clentaminator present in the game, this is actually quite easy, if a bit time consuming and expensive. Dig hellevators every 120 blocks across the area you want to purify. With the Green Solution equipped, float down the each hellevator twice, first spraying to the left, then to the right. I recently did this in one of my worlds, and it worked perfectly. I'd recommend checking every so often after you're done to make sure you didn't miss anything. This will also turn any underground mushroom biome to jungle, so you may want to go back when you're done and convert it back with Dark Blue Solution.

When I did this, I left large swathes of land from the surface to the underworld converted, and even brought in Crimson from another world so I had all the biomes present. Just make sure that if you do this you put wider shafts (I left six blocks) around the corrupted/crimsoned/hallowed areas so they don't spread.