As I understand it, there already exists a filter that prevents posting links in the first N posts (cannot remember the size of N now...)

The spammers are using a specific tactic to get around this;

• 1.) create the first post, with no links. This gets accepted.
• 2.) edit the posting, removing the whatever content it had, and inserting the link

This gets around the forum link filter as edits of own posts cannot be automatically checked, there is no hook in the formu SW for that.
Just proves how ingenious these spam-monkeys are...