I recently got a message from Youtube.com asking if I wnated to enable Desktop Notifications. I honestly did not know what this meant so I allowed it to see what would happen. I soon found out as a little pop up message appeared in the bottom left hand corner of my browser.
It's not really intrusive or anything on my system and I decided to keep it enabled for now. But I still wanted to know how to disable it if later I change my mind.
With Opera,Click on the Menu button, top left hand corner or press Alt + P. If you have the sidebar open on the left, just click on the Settings icon, looks like a wheel or sprocket.
Down the left hand side then click on Websites and scroll down to the Notifications section.
You'll see three options:
Mine is currently set for: Ask me when a site wants to show desktop notifications (recommended). I think it's the default. You can manage the Notification exceptions by clicking on the Manage exceptions... button.
This opens a simple editing tool for changing your notification requests. Hovering your mouse over any site listed will bring up an X in the right hand end where you can just delete your choice. Otherwise click on it to either edit the site name or change the behaviour.
Clicking on the option under the Behavior heading gives two choices; Allow or Block. Your pick. There is also a box towards the bottom that allows you to add a site manually. When Done click on the Done button.