Guide:Maintaining world purity

Corruption
Once you enter hardmode, things become a lot more...well, hard. After defeating the WOF for the first time, the rate which Corruption spreads increases dramatically and is no longer stopped by mere sunflowers anymore. It will, if you let it, consume everything. Although there is no sure way to get of corruption, especially with the flying Corrupters which spit corruption, there is a way to contain it. The Corruption cannot spread over certain blocks(Dungeon Bricks, Wood Blocks, or any player made bricks for that matter), giving any player rich in resources a way to box up the Corruption.

The player has a few tools at their disposal.
 * Corruption and Hallow only spread to grass from infected grass. Placing stone or wooden structures at least five blocks wide will prevent the biome on one side taking over the other side.  Note that corrupted thorns break this rule, so give the structure a little bit of height, too.
 * The Jungle is immune to the Hallow, but not Corruption. A buffer zone of Hallow on either side in the cavern layer is strongly recommended if the Jungle is to be saved.
 * The only advantage of a normal Forest over the Hallow is that it's safer to live in. There are no drops that can be acquired in a normal forest that can't be acquired elsewhere.
 * The Dungeon is entirely immune to everything and provides a natural barrier, which may keep one edge of the map perfectly safe in many cases.
 * Floating Islands are safe, for what that's worth.

Trying to save the entire Cavern layer is a lost cause. It is mostly continuous stone, all of which is susceptible to Hallow and Corruption. Given that, you're best off simply plotting which of the two (hallow or corruption) you want in a given area. Placing Ebonstone and Pearlstone prior to defeating the WoF can make the process run smoothly.

If you are entirely dedicated to saving a section of the Cavern, it's possible to dig shafts from the very surface to the bottom of the underworld that are four squares wide which Corruption ad the Hallow can't cross. Note that if the players destroys even a single Demon Altar, they risk infecting this quarantined section anyway.

Trying to save natural deserts is probably a lost cause. If any sand touches the cavern layer, the entire desert will become infected very quickly. If the player wants a neutral desert, they are probably best off making an artificial one. This is not especially difficult to do.

It's possible, though time consuming, to reclaim a section of the surface by razing every living thing in it to the ground and then planting grass seed.