You need to disable the autoupdate tasks in the Task Scheduler too if present.
Then, for extra good measure, you need to block Opera's autoupdate servers.
Also, some programs like Kaspersky can update Opera for you automatically. If you have a program like that, make sure its program updating is disabled.
If you get Opera via some package manager like Chocolatey etc., you'll want to make sure it doesn't update Opera on you.