@johnhenworth
Go to the Opera directory (C:/Program Files/Opera/) and remove the current version 100.0.xx
Download and install version 99.0.4788.88
After installation, do not start the browser, just re-enter the Opera directory with version 99 and rename the opera_autoupdate.exe file to, for example, opera_autoupdate.exe.OFF
Launch the browser. It will no longer download updates.
This method also works with earlier versions of Opera.