Is there any way to block automatic IOS updates?
Google-fu would seem to think the only way is to keep your device too full to take the download.
You can prevent it from downloading over 'mobile data' but not if it can see WiFi.

To prevent mobile downloads, go to Settings > App &amp; iTunes Stores &amp; disable the Use Mobile Data option.
The specific types above are to prevent the phone downloading other data types at all - meaning you will need to update via iTunes on your computer.  

None of those options will prevent an iOS update from downloading [even Updates, which you may think would be the one] but the Mobile option will prevent it using your phone's data plan.