In short, you can't stop the download once the OS has been told to download it (normally several taps are needed to get to the settings and initiate the download, but perhaps there's a bug where it starts in some rare instances uncommanded).
First of all, I highly suggest you update to iOS 6 because developers are constantly dropping support for iOS 5 for great developer tools that are iOS 6 specific. As for how to stop auto updates, after 45 minutes of searching, I think I found a solution. Doing this will also disable automatic downloads.
Here is what you (or others in your situation) can do:
Navigate to General > Store on your iPad. Next, turn on the switch that is labeled "Use cellular data". That will help most, but in your case, you mentioned turning off WiFi - so if you are concerned about that bandwidth, you'll have to keep networking off until you get to a network where bandwidth or cost for traffic isn't an issue.
This doesn't actually stop the download permanently, but will tell the OS to not burn your data plan and wait to complete the download over WiFi. Now, your phone will retain that downloaded OS image until such time as you erase the device and restore from a backup (where the download is not backed up and will be lost) or apply the update.