By not updating you are putting yourself at risk. Seriously, update the device as your backward thinking is more harmful than helpful.
first off, by that line of thinking, everyone should immediately replace their phone as soon as it no longer supports the latest update.
second, i've had now had numerous experiences with devices that became absolutely miserable to use for their final supported os versions. i'll make my own choice if its worth it to me to endure a frustratingly poor experience for that last patch, thanks.
to the original discussion: i did test deleting the download from storage, and it has not re-downloaded yet, which is heartening. thanks, C DM, and others who suggested that. i would still prefer the ability to simply disable auto-download in the first place. i think its an appropriate compromise that they leave the little (1) symbol on the settings icon to remind users that its available if they want it.
i hope the op has some luck getting his to stop re-downloading, which i hope is indeed a glitch rather than the expected behaviour!