I completely agree! This is one area Android really needs to mature in and provide a native way to control OS updates (ex. like Microsoft does with Windows where you have the option to set OS software updates to: Automatic, Notify only, or Disable). This is especially aggravating when you've bought your phone outright and own it straight up (only pay the carrier for service). In that case I feel the carrier (looking at you Verizon!!!!

) has absolutely no business dictating an update to me. It's MY device and I should have sole discretion whether it gets an update or not. But I digress...
You can disable the OTA updater without root as referenced in this XDA
thread. I did this a while back (Dec.) to stay on KitKat and haven't seen an update notice since. The updater can be re-enabled again if you wish to take OTA updates down the road.
Since that XDA thread has been closed (apparently the dev is no longer providing support) and no longer lists the file for download I've attached the original zip file that I used here. At the time I used an old Windows XP computer and didn't need to download/install any Samsung drivers, just plugged the phone into the PC (using the data/charging cable that came with the phone), enabled Developer options (by tapping it three times under settings), enabled USB debugging, extracted the zip file, then ran the "Block Knox and OTA" bat file. Done.
Edit: looks like this
Debloater tool replaced the previous bat file stuff and why that original dev stopped supporting it and the thread was closed. Either process can be used though to disable the OTA updater.