A few guidelines to keep designated areas safe:
1.) If you're being chased by corruptors (the big guys that spit at you),
do not try to escape to spawn. That spit, if it comes into contact with a corruptible surface, will corrupt it, and that little patch will spread like wildfire. Ideally, kill any of those dudes chasing you before heading back. If you're not able to do that, either head underground, die, or teleport home.
1a.) If you opt for the final option listed, be sure to maintain a good distance -- at least 800 meters -- before 'porting. The despawn distance on Corruptors is pretty annoyingly large, and I think we all know how far the baddies of the corruption are willing to chase you down.
2.) Be a cool person and use either purification powder or vile powder on corrupted and hallowed areas, respectively. We want to keep our home grounds neutral, and that's a pretty freakin' huge task. We all need to pitch in. At some point, we'll put a stash of both kinds somewhere so we don't have to buy or make the stuff.
3.) For the safety of you and any NPCs that might happen to live with you, please avoid building with corruptible materials, especially near either of the spreading biomes. The list is actually pretty small and it's fairly unlikely you will want to build anything out of this stuff, but the list is as follows:
- Dirt
- Mud
- Stone
- Sand
and obviously don't use
- Pearlstone/Sand (Bricks are fine)
- Ebonstone/Sand (Same deal; iridescent bricks are fine, too)
4.) We ought to construct an outer wall at some point; something that goes some 100 blocks underground. Keeping the underground corruption from spreading is a large and futile task, but if we take the right steps, the surface world is nothing if not manageable.