If you go to your actual notifications page, you should see a button in the top right corner that says "Mark all as read," it will clear all of your notifications. You can also go to your settings page, scroll down to where it says "Notifications" and then uncheck the boxes of the notifications you no longer want to receive, then click "save."
In your settings their's a whole block of boxes you can (un) tick with redard to notifications... I've modified those settings to only get the same notifications I received before. Fortunately since I guess you find yourself bombarded with loads of notifications? :o
just hover your cursor on that orange bell thing, click them all on a new tab. When all opened then right click on any tab then click 'close other tab'. That will close all of the tabs opened and your notification is gone.. Then go to setting and sort out your notification preferences to prevent GaG from notifying you again..