They're just spam bots, fully automated.
I run a couple of Drupal sites and I use the fantastic
Mollom service. Every comment posted is checked for spaminess. If it's thought to be spam, the poster is given a captcha. It works great, because most users don't even know it's there (they never see the captcha) and if they do get one, they can fill it out. No spam anymore.
Sadly there isn't a Mollom plugin for phpBB, I know, I checked.
It'll get to the point where a human can't keep up deleting it, or it'll be a fulltime job.
But thanks Kenward if it's you cleaning it up, it's a thankless job!