For certain services iOS 6 now allows you to restrict your alerts to only users that are already in your contact list.
By default, new Messages, Game Center requests and Shared Photo Stream alerts will appear whenever anybody sends you a request or invitation using any of those services, however if you find yourself getting bombarded with alerts from total strangers you can easily turn these off now with a quick trip over to the Notifications section in your Settings app.
Simply choose the appropriate service—Messages, Photos and Game Center all support this feature—and scroll down and choose “My Contacts Only” under the Show … Alerts from heading.