Just wanted to mention a little bit about IM Spam, which I hear is now being called "Spim". Sometime in 2002 I got my first ever IM spam. For about a week I got a bunch. Then for a year I got none at all. Then, perhaps a month ago, I started getting them again. Lots. A bunch. Well, nothing like email spam, but maybe 10 a day. And they are much more annoying that email spam.
I have my IM client (iChat on OS X) set to ask me whenever someone not on my buddy list sends me an IM rather than just showing it to me. So at least I don't see the actual messages. I think ONCE in that time over a year ago, in a week moment I actually looked at one... and egads... I accidentally clicked on it! So I know I am on their lists forever. But when I got none for such a long time, I was happy!
I of course block all addresses that seem like they might be spammers. Things like goodlinda5675t7823 or whatever. But of course they never reuse the same one anyway I'm sure (or do they, dunno, I'm blocking them). But I already accidentally blocked my sister once when she got a new screen name. She IMed me to tell me, I thought it was a spim and blocked her. Oops.
I am getting to the point though where I am seriously considering changing the setting to only allow IMs from people already on my buddy list. If they keep coming, I guess I'll do that. But that sucks.