Has anyone managed to successfully block Yahoo! Messenger via a hardware firewall? I have MSN and AOL blocked, but I've been reading up on Yahoo and what other sources are saying is that it can't be blocked due to the fact it looks for open ports if it finds that it can't communicate on its usual ports.

That seems odd though, you can usually block just about anything at a firewall as long as you configure it right. Can Yahoo really be that crafty? So, I'm asking the people who know at AO (oh, look at me, I rhymed!). Is there a way to block it with a hardware firewall so that it doesn't find a way to wiggle through?