I just waited until I received the notification again, pressed "more info", then removed the battery. I waited about one minute and replaced the battery and turned my phone back on. The Ice Cream Sandwich OTA notification never came back UNLESS I manually checked for updates. Then it would come back and I would have to repeat the same procedure, but it ALWAYS worked.
This sounded a bit whacky to me to be honest, no offence. But I used your idea and tried something simple:
[Please note, my I9300, the phone I'm working on in this reply) is already updated to Android 4.3, I've overcome any wireless issues I had (from memory - that was the main issue with 4.3 wasn't it?) but I can still choose the apps I put at the bottom of my home screen so I'm not limited to only being allowed to have Phone, Contacts and Messages there and I still don't have to press two extra buttons before moving shortcuts on my home screen, which are the reasons I don't want this update. You see, I believe this is a post-update to 4.3 that I unwittingly allowed on my GT-I9305 because after it I could no longer choose the Apps I put down the bottom of the home screen and I had to press two extra buttons to move shortcuts. I am also assuming it is the same update for the GT-I9305 and the GT-I9300 which is why I am desperately trying to avoid accidentally downloading and installing the update on my old I9300, which I'm actually thinking of going back to despite the cracked screen. I mean, no FM Radio because of something to do with the antenna in the 4G I9305? That's a serious retardation in the evolution of the Galaxy S line if you ask me. Then there's the newly apparent dysfunctional relationship between my I9305's bluetooth, audio jack and external speaker. And seriously; don't get me started on all the beeps and noises these phones make when all the volume sliders are turned down, which can only be resolved by a very cryptic, overly lateral analysis of the settings, downloading a myriad of apps each one resolving one problem at a time and, heh, rooting the phones, as if they're not 'rooted' enough already (Australian slang for f@#$ed)
Most likely a combination of the three. I highly doubt my next phone will be a Samsung. Cracks appear on these screens faster than that crack that appears when your plumber's bent over trying to fix your kitchen sink. But I digress! I think the two peeves related to the homescreen are only relevant to Australian consumers using the International version of the phone - apparently the result of a deal made between Samsung and Telstra - though I am hoping this is a universal fix to the main problem - the update automation dilemma].
STEP 1 (just don't get excited coz I don't know the results yet)
I long-pressed the Notification and clicked on App Info, then I just went ahead and cleared Cache in which there was 8.00KB of information (could it be the Update.zip file that you can't find unless you root your device? I guess it would be about half a GB for 4.3 from what I'm reading wouldn't it?) Then I cleared 228KB of Data (or is that the Update.zip file?). Show notifications is ticked and greyed out so you can't change that. I went ahead and clicked Force Stop and clicked Ok.
I went to Settings>More...>About Device and clicked on Software Update again. I already had Auto Update unchecked but I went ahead and clicked Check For Updates. An update was downloaded as evidenced in the Notification Tray just like the first time it downloaded against my wishes after a hard reset and going through the phone setup procedure.
I long-pressed the Notification, scrolled down and clicked Later and I was asked if I wanted to postpone the Update for 30mins, 1 hour or 3 hours (I'm in Australia). I pressed 30mins and Ok.
I repeated STEP 1 and now I'm waiting to see if in half an hour that notification pops up again. We shall see...
I am guessing that since another update downloaded, the cache I cleared was the original download. Since I have Auto Update unchecked in the Software Update options, I'm thinking that I may not be asked to download OR install the update again until I manually Check For Updates again, which I plan on never doing.
It seems to occasionally take pulling an all-nighter, googling answers to my problem, lots of reading and then some extreme lateral thinking to actually give the system a chance to do what it's supposed to do using its natural (unlimited) functions, sub-functions, menus and sub-menus, long-presses, short-presses, checks and unchecks etc. to maybe perhaps find a simple answer that makes Samsung customer service representatives look galactically stupid.
Some good news - after 30 minutes nothing happened. The notification did not return and I wasn't asked to download or install anything.
Some bad news - I just powered my phone off and on and the update was automatically downloaded again and I was asked to install it and I have deleted my "Here's to a promising future...one can only hope" line that I intended to close with.
I went ahead and repeated STEP 1 and selected 30 minutes again. I'm satisfied that I won't have this problem until my battery dies or I restart the phone again. That's as good as I'm going to get right now. for an unrooted phone. So if anyone would like to turn their attention to how to prevent the automatic download from occurring straight after powering up, I would be most grateful.