The spammers are creating accounts faster than someone can approve them, despite the captchas. So trying to approve them would amount to an DoS on our own new users.

Well problem is: New users couldn't use the forums/questions the last days anyway since no one could use them. And I would assume that if their messages aren't coming through, the spammers will stop, or at least reduce in number: Their goal isn't to DoS the mbed new-user queue, but to get their crap on the internet.

This spam seems to stop. Because reCAPTCHA has been embeded in signup page.

Stephen, anything else to enable? They seem to be back today

If that was the case, they would just create an account to answer the question, or just create more accounts. If posts were limited to 1, they would create more accounts.

Then also limit based on IP address. A slightly higher limit maybe, say 5 in the first 24 hours, due to groups of students or project teams all starting on the same day but some sort of limit.

That aim isn't to make it impossible, the aim is to make it more hassle to spam here than to spam somewhere else.

@Stephen, to put it non-diplomatic: I have another solution that does not work, and that is the current solution.

I don't know how they are getting past captchas, if they do it manually or OCR, but for sure it will be more intensive for them than making a new topic/question. So if they have to make a new account all the time I assume it will cut down on the spam.

The easiest solution I see is restrictions on new accounts (and of course lift them as soon as possible to not hamper new users, but currently new users are also hampered by all the spam).