Well, why not use Akismet ? It's pretty powerfull : it analyses the content of every post of given groups (such as newcommers) and if it detects a spam known to his database, it ensures it won't be posted.
I know there are mods for phpBB (
here and its
github seems still active : last update 4 months ago compared to other phpBB mod, it's up to date.
)
Well, anyway, I'm no big fan of phpBB, to have been working with phpBB, myBB, vBulletin and IPB, even if it's half way between a kludge and a Heath Robinson device, I'm still finding the later (IPB) more supple and easy to customize as the others. But that may be because I've got used to it. (It's the one I used the most, with myBB too)
Anyway, Spam is always a big issue. Sadly.