You did not mention the platform you’re using in your forum.
Anyway, I think this might be of help for you.
<a href=”http://www.stopforumspam.com/”>Stop Forum Spam</a>
If you’re using phpBB (you should), you can try <a href=”http://lithiumstudios.org/forum/viewtopic.php?f=31&t=941&sid=a2262036f1e1ffe6c2b7fda5ee19430f”>Anti-SPam ACP</a> which have a good reputation.
Besides this, there are some basic procedures that should be implemented:
- Always use the latest version;
- Implement a registration activation;
- Use difficult Captcha images
If you’re using a decent router, you can always include an in-bound ACL that only allows certain IPs based on their IANA-provisioned IP addresses. Blocking IP from other registeries and only allowing, say, ARIN-based IPs works great…