Robert, On my work forum I ended up asking specific 'product' related questions instead of using the Captcha style registration questions. These days most spam accounts are created by humans rather than bots so it's the only way.
I still get on average 30+ spam account creations a week. I've become pretty good at spotting them over the years based solely on 1st post, email address and user name
I've turned on moderator approval for new members which has fixed the issue. We'll try playing around with the sign-up form and if we can get that to adequately filter newcomers we can revert to auto joining.