Hopefully have a few less of these now (at least for a little while). I've set the captcha test to high difficulty for new registrants, I've also blocked one IP address range that was generating a lot of registration requests.
Some posts were going in to the news forum so I've tweaked permissions so regular users can't post new topics in there.
As soon as I've seen a demo of this new puzzle test thingy for registration I might add that on - who knows maybe spammers are crap at maths