Would it be possible for Atlas to filter out (more of) the spambots? These things are clogging up this forum, and do some annoying over at the Ars Magica forum too.
I suggest you add a simple non-standard obligatory question to the registration stage, if at all possible. Something unique to Atlas Games, so that it won't be worth the while of spambots-developers to overcome. Simply adding a field saying "Text Comprehension Test: Please enter the words Atlas Games in the following field" should, I suspect, relieve the pressure. (Perhaps a more convoluted sentence, if that doesn't work.)
Changing the CAPTCHA you are using is also possible of course, but perhaps more difficult. I have no idea how hard it is, but perhaps whatever program you are using has other CAPTCHAs available? A better text-distortion could help, some other mechanism (audio-recognition, image manipulation...) would be even better.
I know the program you use might not make any of this easily possible. I'm just suggesting in case it does. The first solution at least seems very simple and likely to be possible.