You did not mention the platform you’re using in your forum.
Anyway, I think this might be of help for you.
Stop Forum Spam
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 registries and only allowing, say, ARIN-based IPs works great…