I had to conduct an operation to save my jungle shortly after I entered hardmode, actually, since the corruption stripe was drying and converting it. Here's what it looks like zoomed way out in TEdit:
http://puu.sh/520s7.pngYou can see there that I also had enclosed the original corruption chasms. I did that pre-hardmode.
Here's the border at the surface:
http://puu.sh/520Bz.pngI built the barrier out of mudstone blocks (1 mud + 1 stone, and you can turn dirt into mud while standing near water, so it's easy as hell to make).
There's rope running down the center of the vertical shafts, although TEdit doesn't show it.
And below the temple:
http://puu.sh/521iR.pngSo you can see how this works quite well.
It can't spread through the underworld's ash, or jump more than three tiles (but can spread through vines/grass). The walls are to prevent vines, grass, etc, the three tile tunnel is for being able to travel through it and for aesthetics (and there's no need to fill it, since empty space stops corruption/hallow spread just as well as uncorruptable tiles).
I haven't bothered to stop the corruption from spreading east towards the hallow, or the hallow from spreading into the ice (I get hallow enemies and underground snow enemies at the same time, and can harvest crystals from the walls, so as far as I'm concerned it's twice as good as just having underground snow or hallow).
I did build another wall from the dungeon to the underworld to prevent anything from getting past that, and my skyhome is floating over the east ocean (just high enough that things still spawn in the ocean. I can jump on the roof to get harpies to show up, and they also swarm around the skyhome during a bloodmoon. Oh, and they drop pirate maps
.)
Edit: If you want floating islands' blocks, you have to take 'em apart to get 'em.