To unsubscribe from a website's push notifications on Opera(Mobile), first head to the website on your device. Then, click on the three dashes on the bottom right of your device, next to the home icon.
Once these options pop up, click on the "Gear" icon at the bottom of the page.
Click on "Site settings"
Click on "Notifications"
Find the website that you wish to change permissions on and click it.
You will now be presented with your device's permissions for this site. You can clear and reset it here, if it is not already allowed.