OP:
There's no compelling reason to upgrade to Mojave unless you WANT TO.
I can think of a possible way to get rid of that message:
1. the next time it pops up, click upgrade (yes, click it!)
2. the Mojave OS installer will download into your Applications folder
3. when the download is complete, the installer will open, but...
YOU DO NOT HAVE TO RUN IT
4. Just quit the installer.
5. Now, you can delete the Mojave OS installer or move it somewhere for storage.
Now software update will see that you -have- downloaded the Mojave installer, and hopefully it won't pester you any more.