1. Block certain usernames. No one needs viagra or freemedicine in their usernames even if it is a health related forum.
2. Censor words. Try censoring common spam words. Many of them would be drug related so you might have less words if you have a health related forum.
3. Censor e-mail address. Not many good members would have @viagraandotherdrugs4free.com as an e-mail address.
4. Censor links.
If you see a spam/porno link on your website, add it to the word filter.
5. Turn on CAPTHA for registration.
The most common spammers are robots. So put them to a stop with a good capcha. Most good forum software has CAPTCHAs.
I hope these tips help you to can the spam!