There is three ways that I know of:
1) Delay or Postpone the Upgrade, the down side to this is that at some time the update will be forced on to the phone without any input from you.
2) Root the phone, doing this will void any warranty that you may have on the phone (not recommended)
3) This is possibly the craziest thing to do. Power the phone off and never turn it on.
@Stargazer72460, Nice suggestions, not really feasible.
On the GS6 I woke up to the update in progress, with no permission, so just need some way to tell the GN4 thing not to do it.
Can be done In Android 4 but was not an option from 5 on where the APP's settings allowed you to force no upgrades, FaceBook was one favorite for not upgrading.
May have to get a 12 year old to write an APP for that.