The one problem with IP addresses is that they can change and, with some individual isps, they change rapidly.  With aol for example, what is one person's IP one day can be a totally different person's the next.  Also, many isps have ip addresses change when someone resets their router, and virtually all can change it upon request, making it very difficult to ban someone without membership requirements for comments, and (although unlikely) it could even cause someone to not have access who has never visited our site before. 

I agree with the need for it, unfortunately there is just no easy way to do it for any length of time because IP addresses are so fickle and can be easily changed. . .

I'm aware of these things, but there are also upsides, as replied in this post.
It's easy enough to make a quick check that it's not an AOL user before banning, and dial-up is gradually fading, at least in the US.

My ISP *used* to change IPs at every boot of the modem; in fact, that was one thing I asked and insisted on a guarantee for, before switching from dial-up to their cable service: DHCP random assignment at each login. But some time later, they changed that. You'd have to leave the modem off for, like, 36 hours before they'd assign your IP to someone else. Now they tell me it's more like a week. They claim users want static IP, which is stupid, because the average user thinks IP is what you do when the nurse hands you a plastic cup.  :D And unless you're running a web server at home, which is a violation of the TOS**, usually, (you should be paying the business rate instead of the residential rate), no home user needs static IP. It's all just one more game to invade privacy by helping the marketers and data-miners track your movements.

(** "Terms of Service", not "The Original Song, lol)

And they will *not* change it on request, though I know a few ways to do so myself (won't mention them here).

Anyway, I think more and more users are switching to DSL or cable, and more of those are ending up with a constant IP (always, for DSL, I think). So the effectiveness of IP banning should be increasing over time, and if nothing else, it's one extra annoyance that makes the troll go to that much more effort, only to see the flame deleted the next day. Like kids painting graffiti on a fence -- if they see that it gets repainted *every day*, they'll eventually give up, and go bother someone else.

At the forum where I'm a global Mod, it's only a couple of mouse clicks to go from any single post to banning the IP of the poster (though you should add the reason for the ban). That's running phpBBv3. Just suggesting that it should be that easy here, and if so, might as well make life difficult for the trolls. Most trolls have no patience, anyway.

For the occasional accidental "false positive" -- banning an IP, only to have an innocent user assume that IP and not be able to get here -- there can be a standard screen, something like: "This IP has been banned due to repeated violation of site rules. If you are not that person, please email XXXX, and we will lift the ban. We apologize for the inconvenience that is necessary to keep our site free of abuse." (The email full headers will verify the emailer's IP.)

Only other way is to require registration to post comments, as is required here (00s), but that's been discussed before, and no one wants to discourage casual or one-time users from leaving v/c at a song they happen to find and like.

Thanks for the thoughtful discussion, Dave, and for your efforts as an editor. - TT.