Unfortunately I'm not sure how effective this is going to be. It will block the download from your home wifi, but as soon as you connect to a public hotspot or the wifi at your friend's house then it will just download anyway.
It will only "auto" download the multi GB file if you are on someone else's wi-fi AND you have hard power connected (USB, adapter plug etc).
It will not auto-download if you are just walking around with the device even if on wi-fi. Now - you may get a notification of it being available when walking around on wi-fi but the file will not actually come down via wi-fi alone.
Most folks get caught by this when charging their device. If you do not put your device in Airplane mode - or better yet - plug in and immediately power it down - you will eventually get caught with a huge file on your device at some point. As soon as that hard power is connected and wi-fi is still enabled - it will try relentlessly to get the file.
I tested this extensively when blocking the iOS7 OTA update last year. I have no reason to suspect otherwise with iOS8 in the queue now.
Sonic