It's been quite a while since I registered here, so I don't remember what type of validation is used. I assume there is email validation, where the user signs up, the script sends an email, and the user must click a link to validate and activate the account. Is there any kind of human validation done by the admins, or is it a fully automated process?
I've run a pretty successful forum for seven or eight years. I've seen a number of schemes used by spammers, and have come up with some pretty successful countermeasures.
We use a standard captcha, but those are only so effective. There are companies in India that exist exclusively to process captchas. If you are an attractive target (high pagerank, high SERP, high traffic, or valuable keywords), your captcha is only a penny away from being broken.
We have a field on our registration form that asks a simple question: "Are you human?" The prospective member is expected to choose "Yes" from a dropdown box. If they fail to choose yes, their registration is put into a holding area. If they don't contact us, they drop out of the system after 30 days. This cuts down on 99% of the spam.
We use the email confirmation like most forums. After that, an account requires validation by an admin. Many spammers are easy to pick out just based on username or email address. I'm not about to approve V1agra or Onl1neP0ker777.
From there, we take a few steps that would probably be overkill for a forum like this, like running their IP, email address, and username through Google to see if they pop up. We're a political forum with specific rules and requirements for participation, and we're not for everyone, A prospective member who has been banned at other political forums isn't going to be able to cut it on our site, so we save us both the trouble and deny the membership.
We also have some restrictions placed on new member accounts, like limiting the starting of new topics or previewing a member's first post. I'd be happy to expand upon them if there is interest.
In any event, the amount of spam I've seen here at this forum has been severely cut down since when I first signed up, and that's a good thing. I don't know, however, if using a blacklisting setup is a good way to address the problem. Blacklists have very little recourse for those who have been blacklisted. You're at the mercy of some third party administrator whose goal is to stop spammers, and they tend to be overzealous in their attempts to stop spam. If you've ever been incorrectly labeled as a spammer, you will know what I mean. I have spent dozens, if not hundreds, of hours having my site removed from blacklists. It is not fun.
In this case, it appears as though the service being used is run by a single person. I can't imagine that getting an improper blacklisting removed is a fast, easy, or pleasant experience. I also can't imagine that it would be very effective, either. The removal form says that you have to be a network administrator to request removal, a typical tactic of overzealous blacklisting services. Individuals are usually ignored, and told to have their network administrator request the removal. If I'm on Comcast and I was just given a dynamic IP that happens to be on some blacklist, what are the chances I'm going to be able to convince Comcast to request removal of my IP just so I can join some random forum? Pretty slim. Blacklists are great in theory, but terrible in practice.
Anyway, sorry for the long response. What I lack in electronics know-how, I make up in my knowledge of forum administration.