Sorry I'm late to the game here but hopefully I can help with some pointers. I run a phpBB 3.0.x board. I just registered on your board to see how your process works.
First thing I noticed is that guests are unable to post which the most important step. No problems there.
Secondly, registration approval wasn't entirely automatic but on my site I require administrator approval of all new members. Spammers and spambots are pretty crafty and can even get around the automated activation link sent by email (provided by email supplied by the spammer is valid). I don't give anyone a chance without human eyes looking at the registration.
Having admin approval does take a bite out of your time but it is one major roadblock you put in the way of any spammer/spambot.
When I get a new suspicious registration, I always copy the IP address and run it through www.stopforumspam.com
and looks for matches. If it's a problem there, I block the IP range.
But I have a nifty little mod still in the way of spammers before I even see the spambot registration. I can't remember exactly where I found it but there is a mod available where you can kill registrations based on time zone entered by the spambot. The vast majority of them select -12GMT which is a zone that is uninhabited. Google it to find it but it wiped 95% of the false registrations.
I even dropped CAPTCHA from my registration process. Beyond that, I began blocking problematic IP ranges (at the server level) that persisted in causing me trouble. Mainly, these IP ranges are from China, Russia and Latin America. This is more of a scorched earth policy but it has wiped out my spam registration problem by 99.9%.
On the phpBB software itself, I have blocked email addresses in the ACP using wildcards. All email addresses ending in .cn, .ru, .ua, .in and a few other troublesome spam haven domains are blocked on my board so registration using these TLDs are automatically prevented. By wildcards I mean blocking *@*.ru or *@*.cn for example.
Granted, my board is focused to a region in the US so I don't really suffer consequences from blocking traffic and email from China, Russia, India, Latin America amongst others so you will have to be creative and find what works for you, your board and your demographic.
You won't and can't stop it 100% but maybe using, modifying and applying some of these techniques will drastically cut down time and bandwidth wasted on spammers.
Another good site for phpBB mods and tips is www.startrekguide.com
. They're not as stuck up as the main www.phpbb.com
forums. Any of these forums do greatly appreciate, though, if you do your own basic research before asking very basic questions.
Hope this helps.