I have the Samsung Galaxy S5 with the older Android version 4.4.4.
I have yet to upgrade to the seemingly mandatory update 5.0.
When the update was first released (like a month ago?) I got a popup saying it was trying to upgrade to 5.0, but there wasn't enough space and that I needed to free up like 2GB in order to continue! (I have a lot of apps)
Not wanting to delete any apps, I just ignored it for the time being. Although, the notification to free up space remained. Later that day my parents, who both have Samsung Galaxy S5s, told me how bad the 5.0 was; suggesting I avoid it if possible.
So I continued to ignore the notification, which remained for weeks! One day it was no longer there, I got worried that since I had recently deleted a few larger apps, that it had auto-updated to version 5.0. So I went to check what version my phone was currently running, and although it remained to be 4.4.4, it started up the download to 5.0!
I managed to stop it temporarily by force stopping it, but it started back up after around a minute.
I seemed to have found a simple workaround to stop it from auto-updating and even remove the notifications!
First, turn off WI-FI
Second, you need to get mobile data usage turned off
Under settings->Data Usage unchecking the box labeled "Mobile Data" SHOULD work for this! However...that wasn't the method I used.
I guess I wasn't thinking to do the above method, instead I did "Limit mobile data usage". The lowest I could move the bar down was 0.1GB above my current data usage. After doing that, I started downloading a couple random apps (while continuing to force stop the 5.0 update every minute or so) until I got a notification that my data usage limit had been reached and thus data usage was disabled!
The auto-update still popped up once again, but after it attempted to download for like 10 minutes it failed.
One final pop-up saying the update had failed and that in order to get the update I need to manually go to System Updates and check for updates!
I re-enabled data usage and turned WI-FI back on...NO auto-update attempts. NO notifications. NO pop-ups.
Hope this helps!!! (sorry for such a long rant for just a simple solution)