Spammers are such a pest. I run a forum based on php bb and have been battling spam for ages; at its worst I was getting about 150 spam messages a day, which scared off even the most dedicated of moderators.
Phpbb provides a solution were new members must verify the text written on an image before they can sign up; the trouble with this is that the Spambots used to send troublesome posts to your forum are now capable of reading the images.
There are a few mods out there that can be installed to combat spam.
Unfortunately because of the nature of the install process for mods, I didn’t have time to test them all; however, I used a combination of the Spam Word mod and a Custom Solution to stop spam completely.
PhpBB is very popular to spammers because there are so many forums using it as a platform; because 99% of the installations are all the same, a robot can Post messages to all of them in the same way; by making some very slight changes to my site, relating to the way messages are posted, I immediately stopped about 98% of Spam.
I changed the text in the Signup link and the name of the parameter used in the query string; for example profile.php?mode=unknowntobots.
I had to also change all of the php files that used the parameter to match the new name.
The above solution worked with 100% accuracy for months, but recently I have started getting a small amount of spam again. I have now added the Spam Words and loaded it up with some common words from the posts and it seems to have done a good job of stopping the remaining bits of spam.
There is also a mod to Spam Words, to integrate it with the Akismet service (Spam Database and web service) Spam Words + Akismet which I will use if my current mods don’t work.