Agree Leo, that's the downside of the shortcut method.
Im using portable(not set as default browser) - so i don't know if for instance if you start from a email-link (having opera set as default browser) if upgrade is executed, im guessing it is, proving your point.
Then would be the question if opera's default startup behaviour can be altered,(with parameters). never checked that.
With the perspective that altering the exe filename, with effective the exe "cannot be found", is not the finest way preventing the update.
Then again.. im again baffeled that such an high end browser as Opera does not have a update toggle, be it in the advanced section or so. That can't be that hard to implement i should guess.
Not hijacking this topic,just discussing in between:
The OP's qeustion was "can someone tell me how do I prevent Opera from auto update"