Some updates, not all,, but some, have bugs that we can't live with (like the S3 not being able to use SD cards, or the MM wifi problem in 6.0.1). There are two ways to prevent updates.
1. Root the phone. The update will reach around 25% to 30%, then "fail". What fails is the check to see if the OS is the original and not rooted (which, of course it is).
2. Use one carrier's phone on another carrier (and make sure the manufacturer doesn't issue updates - Samsung does this on F model phones, like the G930F).
The notices still keep coming, but the phone's OS won't update. (To turn the notices off, find the update app once the phone is rooted and disable it.)