Solved Windows 8 Messenger block annoying spam messages

How can I disable notifications from people that aren't in my contacts list

Unfortunately you cannot block the annoying spammers from inside your messaging app. But you can do it from within your messenger/ email account online. Just get to your contacts section and click "manage" and then go down to "view invitations." You can then choose to block and reject the people you don't know. Maybe Microsoft will have an update in the near future.

Problem does not seem to exist after the move to skype.