How can I block the "Update to Mojave" prompts in High Sierra? I have a mission critical Mac that we cannot upgrade because a program that we need will not run on Mojave... and I get prompted nearly every day to update to Mojave...
Or as @martyjmclean wrote:
System Preferences > App Store > uncheck Automatically check for updates
The reason you're being prompted to update is because there's an automatic check for updates. Turn the checking off, and the prompting to update should stop. If it doesn't, or if it's already unchecked, then post again describing the exact situation, maybe a screen shot of the checkbox.