Rename "opera_autoupdate.exe" to "opera_autoupdateoff.off" in the "autoupdate" folder in the install folder. The install path is shown at the URL opera://about.
Open, the Windows Task Scheduler, select "Task Scheduler Library", select each Opera auotupdate task and disable it.
You can also block Opera's update servers for good measure.
If you have any other programs (like Kaspersky) that update all your programs for you, make sure to disable that option in them so they don't update Opera.
See https://www.reddit.com/r/operabrowser/wiki/opera/disable_updates_windows/ for more info.