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
Apple Releases Tool To Report iMessage Spam Ahead Of iOS 7
Apple has today introduced via its website a new tool which will allow you report iMessages address for spam in iOS 7.
To report unwanted iMessage messages to Apple, please send an email with the following details to: firstname.lastname@example.org
- Include a screenshot of the message you have received.
- Include the full email address or phone number you received the unwanted message from.
- Include the date and time that you received the message.
This feature will not only be available for iOS 7 users but will be available for users that are using iOS 5.