Honestly, I think it's the most advanced solution against robots.
I've searched the Captcha on some famous websites, I attached the result at this post.
We can see Microsoft, Apple, Wikipedia, Yahoo!, Facebook use them with a button to listen to the code orally. It certainly exists an addon with reCaptcha for vBulletin for the signup process.
Also, I found the solution of SunBlog is quite interesting and pretty easy for lambda users, but maybe less efficient against bots.
I didn't remember if we have to activate our account by email after the subscription on this forum. It would be a good idea to activate it if it's not the case.