Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
iOS 7 Allows You To Finally Block Phone Numbers
It really seems that Apple has listened to its users after ignoring them for the last couple of years. Users have been wanting to be able to block certain phone numbers from being able to call, message and FaceTime them.iOS 7 now allows you to do that, the function can be found in Settings > Phone > Blocked or you can also can be found at the bottom of each contacts page.
This is certainly going to have a big effect on them who have annoying ex’s who don’t want to give up.