If you haven't yet been hit by iMessage spam, you're lucky: It's awful. And it's even worse when you realize that the spammer can know with certainty that the unwanted message really was delivered to you. As first noted by MacStories, Apple recently posted a way to deal with unwanted iMessages in your inbox, which we first covered at the Hints site.
Unfortunately, reporting iMessage spam to Apple is a multi-step process, and takes longer than simply deleting the unwanted message. On the plus side, though, taking the time to report such spam ought to help Apple cut back on it overall.
When an unwanted, spammy iMessage arrives, first take a screenshot. (If you received the iMessage on your iOS device, press the Home and Sleep/Wake buttons at the same time. If you see the spammy iMessage on your Mac, use Command-Shift-3. The screenshot will go in your iOS camera roll, or on your Mac's desktop.)
Apple also needs to see the full email address or phone number of the person you received the spammy message from. You can either screenshot that data too, or copy and paste it.
Once you have all those details assembled, compose an email to [email protected] (If you receive a lot of said spam, like I do, it might be wise to save that address as a contact.)
Attach the screenshot, the details about the sender, and include the date and time you received the message. You won't see immediate action, but with luck, Apple will pool these reports and eliminate spammers from its iMessage network.
And if all this seems like a lot of work, remember that come iOS 7, you'll be able to block unwanted callers, texters, and iMessage senders yourself.