@gdstuart I have this problem too. It's very surprising and unreasonable that there is no apparent method to sign out of old devices that you can no longer access.
I reset my computer and re-downloaded Opera, only to find that there's no way to communicate to Opera that the previous system no longer exists. I can't remove this 'other device' and I can't delete it from the Tabs Sidebar.
Please get onto this, Opera.