Mac storage basics · macOS updates and installers
How do I stop my Mac downloading macOS updates automatically?
Short answer: In System Settings → General → Software Update, click the info button next to Automatic updates and turn off Download new updates when available.
Open System Settings → General → Software Update and click the info button next to Automatic updates. Turn off Download new updates when available (and the install options if you prefer to install by hand).
Your Mac then only downloads an update when you choose to install it, so a large download can't arrive while you're short of space. Keep installing security updates regularly, though — turning off the automatic download just puts you in charge of the timing.