Thanks Claudia, appreciation is appreciated!
The captcha device on sign-up is extremely complex, and unlike the previous forum I am not sure this one is readable by machine. Accordingly I am wondering if the spammers are just employing teams of workers to create accounts by hand - I've heard of such a thing before. I wonder if it's possible that they can do it quickly enough to make it cost effective? The only further step we could take in this direction is to require a captcha per post, but since they often use an account once or twice and then dump it, this doesn't seem worthwhile - and meanwhile it creates accessibility problems for genuine users.
Today I have taken to blocking IP addresses in the form x.y.*.* - much more aggressive, but will foil them faster. We didn't have this facility in FUDForum.
If that fails to get a handle on the problem, I'll ask Fabien to install the Akismet plugin - it's available for phpBB apparently