For all those people declaring that they don't receive spam on iMessage, good for you.
I receive spam on iMessage several times a week. I block the senders every time, but then they just use a new email account.
It's horrifically annoying.
Definitely recommend that those of us who are actually receiving intrusive iMessage spam, do the following:
Settings > Notification Center > Messages (scroll down to bottom of page and select the "Show Alerts from My Contacts Only" option)
Time will tell if this actually works. Good luck!