How do I protect my world in Terraria from corruption and hallow [duplicate]

If Hallow is present in your world, then you are already in hard mode (so it's a little too late to prepare for it). That said, you can still contain both biomes if you want. Here's what I do to prepare for hard mode when I'm playing Terraria:

1. Dig trenches around all existing Corruption / Crimson biomes to prevent them from spreading. The trenches should be at least three blocks wide / tall (Corruption and Crimson can't cross chasms wider than 3 blocks). Some people like making barriers of impermeable blocks (clay, wood, etc), but I simply prefer air gaps as they are faster to make. Plus, they provide maneuverability around these biomes later in the game.

2. Once you enter hardmode, and the Corruption / Crimson / Hallow strips have appeared in the world, dig a hellevator on either side of the strip. These vertical shafts will keep these biomes from spreading too far to the left or right.

3. It's very difficult (without the aid of a map editor) to completely reign in the spread of these biomes, as each time you destroy a Crimson / Corruption altar, a random block of Hallow / Corruption / Crimson will be created. This "seed" will spread quickly, and it's impossible to know where they all are located.

In the long run, don't worry too much about the spread of these biomes. You need items and materials from both for end game stuff, so you want some of each biome to stick around. That said, it's imperative to contain any biomes that spread too close to other sensitive biomes (the jungle being a prime example). Once the Corruption / Crimson has spread into the jungle, it's nearly impossible to revert it to its pristine state. The same can be said for the two oceans.