Spambots and meat-spammers (aka humans paid to post spam on forums and blogs).
Our automated anti-spammer system blocks anything it considers suspicous, including registrations from certain countries from which we have had the majority of our spammer registrations.
If a legitimate users is inadvertantly caught by the system, they usually contact us and once we've verified that they are legitimate, we'll approve their account.
FYI - they are not automatically banned - they are put into a "moderation" queue for manual approval. The problem is that we can't distinguish between spammers and non-spammers in the queue, so we wait for the user to contact us so we can approve their account.
It is unfortunate we have to do this and I regret the inconvenience it causes to legitimate members - but that's what we are forced to do to keep the forum as spam-free as possible.