Begrudgingly, I've added reCAPTCHA to the sign up form. This is just for now, and only for free accounts, to try and reduce the number of spam accounts being created. It's just too much work for me to keep them out manually, so I think this will help
So far so good on this -- seems like a much higher ratio of real users vs. spammers, and from what I can tell, no automated accounts getting through.