Have you tried
Ozzmodz login spaminator add-on? It works great for me. I never have a problem with spammer registrations. I do not use any forum of captcha for registration.
Looking at the "spaminator" log on my forum, this add-on has blocked 25+ attempted spam registrations in the past 24 hours....and hundreds over the past 12 months.
Additionally, I use the
Geoblock registration add-on by Simon Hample which allows me to block all access from certain "problem" countries such as Russia, China, and a few others....who do not need any access to my forum.
Using these two add-ons, I never have a spam registration.