How can I halt the spread of corruption after winning hell?

You just gotta make sure the Corruption is contained (i.e., has nothing to spread to). Any Corrupted block can spread via these blocks:

• Exposed Dirt and Grass
• Stone
• Sand
• Jungle Grass
• Mud

Additionally, it can jump up to three blocks (that is, clear 2-wide gaps). Therefore, digging a trench lined with incorruptible materials should stop it. However, preserving the natural biome isn't very practical in the long run, for several reasons:

• The Corruption can spread underground via the abundant Stone, and possibly tunnel under your trenches and corrupt everything in a heartbeat.
• Corrupted grass can develop thorns, which could reach across your trenches and start corrupting.

Creating a Hallowed biome really is your best bet against the Corruption. It spreads just like the Corruption, but they cancel each other out. Corrupt blocks will not infect hallowed ones and vice versa.

It's still rather dangerous to be in, but I believe it is the lesser of two evils.

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.