You may still have to wait till iOS 7 to block users rather than specific messages, but you can now block and report spam on iMessage. It's a little baffling that it's taken this long, as the other major IM providers have provided this capability for years. It isn't that there's no spam on iMessage, but perhaps it finally reached a tipping point. (Perhaps because of the Denial of Service attack against iOS Developers this Spring?) The method that's provided isn't exactly simple or easy, but at least it's a start.
Apple's support document provided the following step by step instructions:
To report unwanted iMessage messages to Apple, please send an email with the following details to: email@example.com
1. Include a screenshot of the message you have received.
2. Include the full email address or phone number you received the unwanted message from.
3. Include the date and time that you received the message.
To take a screenshot:
- To take a screenshot on your iOS device, press and release the Sleep/Wake button and the Home button at the same time. The screenshot is added to your Camera Roll album.
- To take a screenshot in OS X, simultaneously press the Command + Shift and the number 3 key on your keyboard. The screenshot is saved to your Desktop folder.
To learn more about taking screenshots in OS X, see article HT5775.
Have you experienced spam on iMessage? Will you start reporting these unwanted messages now, or will you wait until iOS provides a more automated method?