It's possible to stop the Unreal Engine Launcher from doing its automatic update whenever you start it. It's not really a software setting or configuration as you might expect but according to Epic, it's merely a matter of adding a flag to the launcher executable shortcut (typically on your desktop). If you R-Click the shortcut and choose "Properties" from the menu, then choose the "Shortcut" tab you'll see the "Target" path location. Simply add:
"-noselfupdate" (without quotes)
at the end of the path, after the quotation mark.
So it would read something like this in the "Target" field:
"C:\Program Files\Epic Games\Launcher\Engine\Binaries\Win64\EpicGamesLauncher.exe" -noselfupdate
Of course, to ever get any of the updates, and they are frequent as you know, you will have to remove the added flag before starting the launcher.