Have we gone up on a search engine results page or something? I don't know why the forum for a rather specific subfandom would attract them in like this.
EDIT: Hm. On one of the other forums I've used lately, new members can't post links (etc) for a set period of time/number of posts after the account is made. Does this forum software have that ability? I've also seen them restricted to a particular subforum until that time passes...if instead of time, you used "when an admin has a chance to approve the account" it might be as effective and at least would keep the spam off the rest of the forum.