you gotta be quick and delete the auto-update folder as soon as the previous build is reinstalled.

You can pass the --launchopera=0 command-line switch to the installer so Opera doesn't open after installing. Then, you can delete opera_autoupdate.exe before starting with having to rush.

Note that while this will break checking of updates at the URL opera://about, Opera will still notify you when there's a new update by putting a dot on the Omenu button at the top left of Opera. To block that, you can block connections to the update servers to see if it helps.

Also note that Opera has update tasks in the Windows Task Scheduler. If Opera ever updates on you despite having deleted opera_autoupdate.exe, once you roll back again and delete opera_autoupdate.exe, make sure to delete/disable those tasks.

Also note that you'll have to delete opera_autoupdate.exe (and mess with the scheduled tasks if necessary) each time you manually update Opera.

Also note that if you have a program like Kaspersky that updates your programs for you, you'll need to disable that so it doesn't update Opera on you.