Ok, I got rid of spam registrations using Q&A but the buggers just post as guests (yes I know I can switch it off but I do not want to).

Any chance you can add Q&A to guest postings?

This would be a handy feature in my opinion. I have coded a similiar hook for a client of mine but I expanded so it includes members under a certain post count too. After this the spam posted at his forum dropped almost to 0.