@space49 You could try sudo apt-get purge opera-stable, if sudo apt-get remove opera-stable does not work.
But you should probably try getting help from a linux-forum as the problem is not opera but linux.
The error message you get from your "application center" is probably something you can ignore for now. I do not know Elementary OS, but I imagine that you can remove the opera-repository from it - then the error message should be gone, too.