This guide is meant for people that just started a forum and want to proof it from those ugly bots that just post useless stuff and take up your board's space, speed, and bandwidth, and use valuable system resources.
I have a few tactics to share with you.
1. Install CAPTCHA - If you don't have CAPTCHA software (or even better, reCAPTCHA) software on your forum, you're setting yourself up for disaster. Spam bots can just register using automated software and spam up your forum before you can even touch the "lock" button or "delete" on a topic.
2. Human Verification Questions - Using custom registration fields, make random math questions or "Are you human?" questions, and then set the board to automatically display the wrong answer (some boards like NovaBoard and MyBB and IPB DO support this, so look).
3. Forum Permissions - Many webmasters overlook this, but after testing the theory, it's the most dangerous vulnerability around. Make the VERY first forum you on your forum (the one on the top) called "Announcements", and disable Member posting and replying. Then, somewhere else, make a forum called "Announcements Discussion", where members can discuss each announcements. The trick behind this; bots usually spam up the first forum they can see, so if they can't post in it, they'll screw up and end up not visiting your site altogether.
4. Grammar - There's nothing a bot likes better than posting in topics with titles like "ipb php mail" or "luv 4 u". When they see that, they spam up the topic. So enforce grammar rules in your forum.
5. Special Software - There are some forum softwares that have special MODS designed to interact with a special server containing the names, emails, identities, and IP Addresses of almost ALL spammers known to man. Offhand, I know that IPB, phpBB and MyBB have this available to be installed.
Follow all the following steps, and you'll be great and doing fine!