Hai provato quanto segue dal sito web di Apple :
OS X Yosemite: Turn off notifications
If you don’t want to see notifications in the top-right corner of your
screen, you can turn them off. You can turn off notifications for
certain apps, or turn off all notifications during specified times by
setting Do Not Disturb. For example, you can turn off notifications
sent from the Calendar app, or turn off all notifications for when
you’re asleep.
Turn off notifications for an app
Choose Apple menu > System Preferences, then click Notifications.
Select the app on the left, then choose None below “[App] alert style” on the right.
The app’s notifications won’t appear on your screen. If you also don’t
want to see the app’s notifications listed in Notification Center,
drag the app down, to the Not in Notification Center list on the left.