How to make captchas impossible to spam

It seems that a solution has been found to counter the spammers who use programmed robots to register on forums and "lay" their advertising messages.

To guard them against these pirates, CMS uses captchas, which are distorted letters so that people can read them, but not software.
But this protection has its limits and software robots now are able to pass such protection, which has only the effect to annoy users, who can not always recognize the letters!

A blogger had the idea of a system almost impossible to break by a software, 3D captcha, easily identified by humans, but not for the computer that does not have embossed capabilities and form recognition that has the human brain.
You can see the practical implementation of such ideas with captchas on the Yuniti site. (Go to register and click on the locations for 3D images).

The result looks like this:

The user must retrieve the three images that are proposed in a panel of 3D images.

The implementation of this system is not complicated once you have images in 3D, which you can make yourself or tap into a stock of images under a free license: the registration program knows the number of the three pictures and that of images in the panel, it is sufficient to verify there are the identical, while the bots have only the images.

Questions

An efficient alternative against robots who register on forums and post advertisements is a script that asks a simple question that the user must answer.
If the list of answers to the questions is personal, bots have no chance of passing the dam.
This system is incorporated into phpBB.