In the version folder in Opera's program files folder, rename/delete opera_autoupdate.exe. Then, it won't be able to update. You won't need the --disable-update command-line switch then.

You can also add the OPERA_AUTOUPDATE_DISABLED environment variable to the system (or just your user) as describe at https://superuser.com/a/908364/6488. That's supposed to stop any Opera from updating.

Further, disable Opera's autoupdate tasks in the Windows task scheduler. See https://superuser.com/a/1169792/6488.

Further, you can block the autoupdate server in Windows. See https://superuser.com/a/1172541/6488.

Renaming opera_autoupdate.exe is the only one that you should have to do though.