Cleaned it all up again, it didn't take long, but we have to sleep some time...
Every restriction we put on new signups affects legit users too - things like manually approving posts or limiting to one post on first day presents extra barriers to entry for new users joining the community. The existing link blocking was very effective when we first implemented it, but still is inconvenient for some new users, especially in bug reports. We need to improve our automated spam detection somehow, but without tightening the restrictions on new users so much that they don't feel welcome or that they can participate as much as they want to.
This latest wave is tricky because it all seems to come from different IPs (I'm banning them all, it doesn't seem to help much), it's circumventing the signup captcha somehow, and they're not posting real links (which makes the whole exercise pretty pointless, but what do spammers care... sigh...). Tom happens to be away at the moment but he's back in a few days and should be able to find a way to cut this out automatically. Until then, we'll have to keep cleaning!