You want to download patches and install security updates on your computers only using the GFI LanGuard server, and stop automatic checks for updates by Windows Update on client machines.
There is no setting from within LanGuard that can disable automatic Windows Updates. This is a setting that you will need to configure in your environment either through GPO or manually. You can refer to our guide on how this can be done on Windows 10 as an example, as the process of disabling Windows 10 automatic updates is not straightforward since the option is hidden.
Do NOT disable the Windows Update service, as it has to be enabled for the GFI LanGuard to function correctly.