I believe you need to reconsider allowing new members immediate access to post new threads.
The current anti spam strategies are clearly not working.
The bots will just post in other threads first.
Seen this happen on many forums, suddenly you see old threads with bullshit answers (broken swedish usually in my case, straight from google translate probably). Few posts later and bot created its spam post.
But y captchas is kinda meh, they stop as many legit users as bots.