I was thinking the same thing lately.
Everyday I come here, I find a flood of spam threads from like 4-5 new spambots who make around 200 new posts a-day each. Which is kind of sad, because I want to open a new topic and ask things, but it gets lost in the sea of spam.
What should I say?
Wipe this whole forum, install a new functional one(this one is sort of broken with the spam going awry and inability to edit profiles); move the database there, turn on spam filter(that should be a default setting). I know larger forums, with a much bigger userbase who NEVER have deal with spam.
There should be an option at registration where the forum generates a code image and the user has to enter it in a box to prove that they're human. Which pretty much solves most of these problems. I'm not entirely sure if phpBB's have it.