1. Last year i discovered that some automated bots have been successfully defeating the captcha on the smf forum system we use, and registering 50-100 fake users per day. I put in place several handmade fixes to the registration page, including a few changes to the captcha to make it harder and use non-standard fonts, and swapping around some of the input fields. The result being that the automated bots that think they know how to solve smf forum system captchas now fail. I also save the failed captcha attempts to a db table so i can look at them, and it's quite interesting to watch the bots fail. for those curious, they also tend to fill in missing fields with the name of a US military branch (air force, navy, army, marines) -- why, i don't know. So hardening the captcha on the registration page and using some non-standard changes to the default used by your forum system so that you stop spam bots from signing up -- this is your first line of defense.
2. But stopping bots wont stop all spammers, some sign up manually. For that, integrating a service like stopforumspam
can be incredibly powerful in stopping spam. whatever forum you use, find an addon that can query stopforumspam or something similar -- that's your second line of defense.
3. Then you can try to make your forum less appealing, by doing some of the thing's app describes that i did, making links nofollow so that they don't benefit spammers much. Personally I don't think spammers pay enough attention to realize this, so it won't discourage them, just deny them the benefits after they do spam
4. The fourth and final line of defense is the human factor. Here's what i did for us -- i wrote a custom email notification thing that alerts us moderations whenever someone makes a "suspicious" post OR profile modification. What qualifies as suspicious is if it's their first or second post, if they are a new member, if they have a url somewhere in what they are changing, etc. The email includes a summary of the change they made, and a quick link to ban them. This is our secret for how we keep the tricky spammers from ever surviving for more than a few minutes on our site
.. This is what allows us to catch the really sneaky ones who do tricks like make a normal post, then wait a month and edit it to add spam links in their old post. Many forums will miss this kind of attack because no one notices the change. So having these custom alerts is a big win.