Apple's Notification Center in OS X Mountain Lion provides a quick way to get messages about various updates, such as new e-mails, calendar events, reminders, and system and application software updates from the App Store. However, regardless of its uses, Apple has somewhat imposed Notification Center on users by making it a persistent service that isn't easily disabled. Since the service does add yet more items to the menu bar and since it does in part directly conflict with current notification options like Growl, some people might wish to turn it off completely.
It is possible to disable notifications by holding the Option key and clicking the Notification Center menu bar icon, or by scrolling up in the Center's sidebar and turning off banners there, but this is only a temporary setting; notifications will be re-enabled the next day. Another approach is to remove applications from the Notification Center and turn off banners using its system preferences pane, but this is also an incomplete solution since the service is kept alive in the background and the menu bar icon will persist, and newly installed applications may be automatically added to the Center.
Furthermore, unlike other menu bar items such as Sound, Time Machine, and Wi-Fi status, the Notification Center is not a true "menu extra" that would be handled by the system's interface server (the SystemUIServer process), and therefore cannot be independently removed or even rearranged among other menus.
While Apple's implementation of Notification Center can be a bit frustrating, it turns out that Notification Center for now is not deeply integrated into the OS and can be set up so you can quit it without adversely affecting the system. The Center is simply a single application in the /System/Library/CoreServices/ folder that is dynamically launched and kept alive for each user's log-in session by the system launcher, a process called launchd. Like other launchd actions, this behavior is governed by a a single launch agent file -- in this case called "com.apple.notificationcenterui.plist" -- in the /System/Library/LaunchAgents/ folder, so there are several approaches you can take to keep it from loading.
The first approach, recently outlined by OS X Daily, involves going to the CoreServices folder and renaming the Notification Center application itself. This will break the path that the launchagent file uses to locate the program's executable, and thereby prevent it from loading at bootup. While easy enough to do, simply changing the name of the program may not be the best thing to do, as system updates may restore its functionality.
The alternative approach is to alter the launch agent file to prevent the system launcher from keeping the Notification Center persistent and active. There are a couple of ways to do this, including removing the launch agent file itself (by moving it to a subdirectory called "disabled" that you can create if it does not exist). To modify the system the least, you can simply alter a variable in the agent file to prevent it from continually launching the Notification Center.
To do this, open the launch agent file in a Text editor that supports authentication (Bare Bones Software's TextWrangler is a good option), and locate the entry key called "KeepAlive" that should be at the top of the file. Underneath this key is its value, which should be "true," but which you can change to "false" and then save the modifications (change only the text, and not the surrounding characters). This edit can also be done by running the following command in the Terminal (all in one line).
sudo defaults write /System/Library/LaunchAgents/com.apple.notificationcenterui KeepAlive -bool false
To undo this, simply repeat the command and replace the word "false" with "true" in the command.
When this edit is done, restart the system to reload the modified launch agent file, and Notification Center will now be disabled. Even so, you can still run Notification Center if needed, by going to the /System/Library/CoreServices/ folder and launching the application from there. To facilitate this you can create an alias for the Notification Center on your desktop, or even add it to your Login Items so it loads when you log in to your account.
When you load Notification Center manually, there is no launch agent control over it, and you can quit it directly if needed and thereby better manage it on your own terms. Unfortunately since it was not intended to be directly quit, the program doesn't have a Quit command, but you can do so by selecting Notification Center in the Activity Monitor and clicking Quit in the menu bar, or by running the following command in the Terminal:
You can use this command directly, or put it in an automator work flow or AppleScript application to create a tool for quickly quitting the notification center.