I can certainly vouch for most botnets being controlled via IRC. I adminster an IRC server and I'm always having to kick them off (I hate botnets). However blocking irc isn't that simple, there is no one port that it uses (there is a default one though). And you can be sure that the botnet owners will rapidly change port numbers on you. Better to make sure/encourage people to keep their machines clean. After all, being part of a botnet and ddossing someone may be bad, but having your personal data stolen via a keylogger e.t.c is worse.
I could wish that IRC admins were a little more proactive about booting botnets off their servers. I often see signs of them on varius servers but no-one seems to take action
Admittedly my server is a small one (we focus on creative writing) and so when a channel with 100+ weirdly named people turns up on it. it's a bit obvious
