Spam seems to come in waves - generally when we start getting a lot then I think Tamsin makes some small alterations and it seems to go back to normal levels again so it could be the questions etc on sign up need a little tweaking. I believe we've also stopped registrations from certain emails that are commonly used by spammers but, as weeble said, there are always ways around the checks so it's very hard to stop it completely without hindering the other members.
We discussed manually approving users, moderating first few posts etc in the past and took the view that, with the nature of the forum (the health section in particular), it was more important for new users to be able to post straight away. Of course, this means that other users do have to view some spam but it's often pretty easy to spot from the titles so, as a normal user, I would have either reported or ignored most of them completely whereas, as a mod, it's easy to spot them to delete them.
I know it's a bit of a pain to see but hopefully not too much of an inconvenience.