More spam than Monty Python

I run a small, low volume site, and I'm getting more spam signups than actual signups by a factor of 50:1. I've configured Captcha, require email verification, have my stop forum spam key put in.

All their posts get caught by the moderation tool, but it's still a huge PITA to manage this. (Nevermind that my staff can't seem to regularly find the spam cleaner because it's not easily visible in the moderation queue.)

Is there anything more I can do to stop spam signups?