Spam is nothing new. But if you are getting spam when using Apple's iMessage service, you can help tackle the problem. Apple has implimented means for you to report iMessage spam. To do this, you will simply need to take a screenshot of the spam message, and send the image to Apple in an email.
The steps for doing so are relatively simply, and only require you to take the screenshot on your Mac or your iOS device and then include the shot in an email attachment. Apple has outlined this process in a recent knowledgebase article.
With the spam message showing, in your iOS device press the Sleep/Wake button and the Home button at the same time, and the screenshot will be added to your Camera Roll album. In OS X, you can press Shift-Command-3 to create a screenshot that is stored on your desktop.
While the OS X screenshot will work, it will include everything on your screen. If you only want to take a picture of the iMessage window, then press Shift-Command-4, then tap the space bar, and you will see the cursor turn into a little camera. Place this over the iMessage window and when the window highlights, click to take the shot.
Lastly, these options save the screenshot to your desktop, but you can instead save them to the clipboard by including the Control key in your screenshot hotkey sequence. With it saved here, when composing your email to Apple you simply need to paste it into the new message, instead of having to access and drag it from your Desktop.
Once your screenshot is created, compose an email to email@example.com and include the screenshot, the spammer's full name, address, and phone number, along with the date that you received the message. Apple will then take measures to prevent this spam from occurring again.
Keep in mind this service from Apple is specifically for messages sent via iMessage. This will be the case for OS X users, but those using iOS devices may have SMS and MMS spam in addition to iMessage spam. For SMS and MMS spam, Apple will not be able to manage it, but recommends you report it to your cellular carrier.