It does. But there are two types of forum spam – automated and human. The CAPTCHA should stop automated spam (i.e. a script automatically signing up accounts and posting spam). It's pretty ineffective here, since phpBB is so widely used and hence the CAPTCHA has been broken. The next version of phpBB (3.0.6) is going to add more CAPTCHA types, including reCAPTCHA and a full API, so it should be a lot easier to customise each forum's CAPTCHA which will improve security. We also have a couple of anti-spam questions added to the registration form (basic maths questions) which have pretty much eliminated the automatic registrations here, since they're unique questions that automatic scripts aren't expecting to be on the registration form.
CAPTCHAs cannot stop spam posted by humans though, which a high majority of forum spam is, since the CAPTCHAs are supposed to be easily solvable to a real person. I would guess this is a case of a human spammer and their account has been deleted, as per forum policies. We actually get these sort of posts fairly often, but they're usually deleted fairly quickly.