Temporarily uninstall Opera GX (don't choose to delete your data when uninstalling if asked).
Then, manually clean up all Opera leftovers (except your "Opera GX Stable" profile and cache folders).
Then, download the Opera GX installer from https://www.opera.com/download#opera-gx, launch the installer, click "options", uncheck "import data from default browser", adjust all other options as you like, and install.
Then if regular Opera comes back again, it means something else on your system is installing it. It could be bundled with some other security software or you could be experiencing an issue similar (but not quite the same) as this issue.