There's too much spam on Yahoo Messenger, and Yahoo seems to do nothing about it. It's all very similar. The accounts names look as if they're generated by the same algorithm. Each message includes one randomly selected emoticon. Finally, and most importantly, each message contains the URL of some web site with the characters of the URL alternating with non-printing characters, as if I was viewing UTF-16 as 8-bit.
Messenger spam is especially annoying because the message results in an open window "in my face". Yahoo seems to be able to handle e-mail spam; I get practically none. So why cannot they handle Messenger spam? I've already set Messenger to not allow Yahoo web sites to show I'm online. So the only option available to me is "Ignore anyone who is not on my Messenger list". It's too bad that I've been forced to set that, but there are plenty of other ways to contact me.
Edit (Feb 26th): I had a Yahoo IM link in my profile on Blogger. I wonder how much of the spam was because of that.