Email authentication is on, it doesn't help. Not allowing links in member profiles is also no an option in SMF.
I tried installing an Akismet (anti-spam) plugin for SMF but then posting would time out. Why? I don't know. Nearly all Wordpress sites on the internet use Akismet and I've never seen an issue where it take 5 minutes to post something.
Both phpBB and SMF are pieces of ���� software. Look how you install so called "modules" you have to manually edit code core to the system or you run a patch that does the same thing. It's garbage and unmaintainable. At least with Phorum, modules are modules, like they are in Wordpress.
I've made the first step toward moving to Phorum, it is installed. Next step is look an feel. After that I need a user and post migration script. After that I could launch but the dice roller won't be ready.
That is weird. I'm familiar with phpbb2. I didn't bother banning people, I just didn't let them link to their website until they had posted. Do you have email authentication on? That helps.
Unfortunately the evil spammers employ real people. So even a good captcha will only get you so far. Do you have one?
Captchas have been getting really hard to decipher recently, even if you are a real person.
On my blog I don't approve users unless they manually email me. Depending on how much traffic you get, that might be less work.