Though you can change this behavior by disabling Java Script, here is how to do this in Opera without disabling Java Script completely and without any plug-in. Earlier versions of Opera had ignored such instructions, but the new versions run JavaScript. To re-enable this Click Opera button and select Settings and then Preferences.

Switch to Advanced tab and select Content category to the left. Click the JavaScript option button and deselect ‘Allow Script to detect context menu events’ and save the changes.