I believe I remember reading that the corruption itself cannot cross a gap, but if its vines grow across the gap, they will continue the spread of corruption on the other side. Could be wrong though. I'll try to find out more.
Edit: Haven't seen anything else to help, so that's all I've got :)
Edit edit: Okay, Got it figured out. Apparently corruption/hallow can jump up to three blocks to affect other blocks. You can prevent them from doing so by digging a 4 block wide channel. You may be able to leave it empty, but if you have trouble with corrupted vines, you may need to either fill it with ash, which cannot be hallowed/corrupted, or wall the edges with stone so as to prevent the vines growing.