You have no idea how pissed I would be if I couldn't get onto CFC because some jerk-wad flamed someone else, and banned the IP range for my broadband.
That being said, it's easy enough to change your IP. I do it so often at work (setting up new 'puters from an image) that I can do it in my sleep. In fact, I think I have, sometimes!
Sabo, what you said last is probably best. Send a cookie. Of course, as you said, this isn't perfect. But it is better than banning a single IP or an IP range. it just affects the one puter. It's not as if there aren't CFC cookies on the 'puter anyways, as I never log in, unless my wife blows the cookies away. So you could just have the cookie put in there, "banned" and have it on the server side so that if the banned user comes in, they send a new cookie to the 'puter with the banned code in it.
Edit: Very frustrating, isn't it? It would be nice if people were a bit more mature in how they handled differing opinions.