Spam filters have been defeated

I feel like I've banned a hundred accounts in the last week.

It is good to see that a new spam account can only create three questions initially, but it's obvious that the Captcha on signup has now been thoroughly broken and we need to reconsider improved anti-spam-user techniques.

I would start with
1) A look at a better Captcha system in the short term (the fact the current one simply isn't good enough strongly suggests that better ones will be broken in the future, so it's only a quick temporary fix)

2) Spam analysis algorithms - blacklisting of new user's postings unless they actually look coherent and vaguely relevant

3) Absolute block on posting of links for new accounts (the main reason they do it is to get links picked up by search engines, so removing that facility removes any incentive to do this). Although I'd consider a whitelist so that links to other questions or Atlassian documentation are allowed through