I use Adium as my IM chat client because it allows me to chat across networks (AIM MobileMe, Yahoo!, MSN, Google Talk, Jabber, ICQ, and so many more) from one app.
Somehow, my MSN Messenger contact info must have gotten out, because I began to get hit every 5 to 10 minutes on MSN with a SPAM instant message. It varied slightly, but was usually something like “I can’t get (something) to work. Check out my pictures at (url)”, or “I saw you on (blah), login to (url) so we can chat.”
Thankfully, blocking this unwanted IM SPAM is easy in Adium. From the menu bar, choose Adium>Privacy Settings… Choose the offending Account, in my case, my MSN account. Then, for Privacy Level, choose Allow only contacts on my contact list. Close the window. Presto, no more instant message SPAM from that account.