While Corruption, Crimson and Hallow spread slowly and only via grass and thorny bushes during pre-Hardmode, they spread very aggressively and to a multitude of different blocks once the Wall of Flesh has been defeated. Defeating Plantera slows the overall spread speed by 50%.

Prior to entering Hardmode, it is recommended to contain the existing areas of the Evil Biome. The simplest step, and often the first, is to dig a tunnel 6 blocks wide completely surrounding the areas. It is also a good idea to build these tunnels around areas of interest, such as the Jungle or the player's base.

As soon as the Wall of Flesh is defeated, two diagonal strips of Hallow and Corruption/Crimson will generate in a V shape. The location of these strips and distance between them is completely random, but if the player has done a lot of previous underground exploring, the affected areas will be very clear. These diagonal strips will begin spreading as soon as they appear, so they can create disorder if left unattended. However trying to contain the spread at this point is a bad idea, as the new enemies can easily kill players at this point, and the tools available are not powerful enough for the task. Hence it is recommended to wait until better gear is acquired.

In order to obtain this gear faster, one might consider smashing numerous Altars in order to find the new Hardmode ores more quickly. However, note that every single Altar smashed turns a random block into its Hallow/Evil counterpart. This will barely speed up the overall spread, but can be harmful if the blocks spawn inside contained areas.

If the Hallow did not spawn on the side of the Jungle, it can be helpful to artificially create one around it to prevent the Evil Biome from converting it.

Once a Mechanical Boss has been defeated, the Steampunker will arrive, giving the player access to the Clentaminator, which helps when "cleaning" massive parts that have already been converted. Note that this tool and its Solutions are fairly expensive, so the player should have the necessary funds at their disposal. See Guide:Making money for further reading about this issue.

If you want to make cleansing your entire world a feasible task, the Clentaminator is an essential tool.

The most reliable way to completely convert or purify a world is to dig multiple Hellevators with a horizontal distance of 116 blocks (the maximum range of the Clentaminator) between each other, then glide down each one using Wings or a Featherfall Potion, while spraying both sides. Additionally, the Clentaminator produces a decent amount of light that can completely go through blocks, mapping out the entire world.

As desirable as completely eliminating the spread might sound, leaving a part of the world with Corruption/Crimson and Hallow is not a bad idea if contained properly, since there are a lot of items exclusive to these biomes the player may need later.

• Thorns can grow from Crimson, Corrupt, or Jungle grass. Corrupt and Crimson variants can grow 6 tiles in all directions, and spread their biome up to three blocks away like any other source, but their spread can be contained with physical neutral barriers, like Clay or Wood. Jungle thorns can spread into range of Corruption, become corrupted, and spread further into the Jungle. If left unchecked, thorns can inadvertently spread the Evil across otherwise safe gaps.
  
• The Dungeon can completely prevent the spread of Hallow and Corruption/Crimson if it reaches down to the Underworld. If it doesn't reach that far, it will at least slow it down.
  
• Floating Islands can be turned Hallowed or Corrupted during the initial generation from Hardmode, or if converted blocks are placed near them.
  
• A drawback to using the Green Solution is that all Mushroom biomes will be converted into Jungle. An easy fix for this is to go back to each pocket of former Mushrooms and spray it with Dark Blue Solution from the Truffle.
  
• You can use Blue Solution, Purple Solution, or Red Solution to completely convert a world to Hallow, Corruption, or Crimson, respectively.
  
• Items that give a chance to not consume ammo affect the Solutions from the Clentaminator. With enough of them, a lot of money can be saved as the player can spray with smaller amounts for much longer.

Source[terraria.gamepedia.com]