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
iMessage Users Hit With Denial Of Service Attack
A big advantage of iMessage for most people is that your can read and reply to message on all your Apple device’s including Mac’s but now this may be to blame for the recent attacks some iMessage users have experienced.
Some users have had their accounts flooded with multiple messages saying the exact same thing from and some messages even claiming they are from Anonymous. The hack does not appear to be that advance at this stage with it just being an Apple script which can be run to flood accounts with messages. @chpwn from Twitter added to the evolving story saying his iMessage account was hit with long stings of Unicode characters which resulted in his iMessage app completely freezing.
Apple has not had the best of luck with security over the last couple of months and the best solution at the moment seems to be for Apple to find the culprits and block their accounts they are using to send out these messages.