How to implement captcha is impossible to hack

It seems that a solution has been found to counter spammers who use software robots to register on forums and "postpone" their advertising messages there.

To protect against such hackers, CMS uses captchas, which are letters distorted in such a way that a person can read them, but not programmatically.

However, this protection saw its limits and software robots now manage to easily pass this protection, which not only annoys the user who does not always have time to read letters!

Then the blogger came up with the idea that it is almost impossible to break the system using software, 3D captchas, easily identifiable by humans, but by no means for a computer that does not have relief vision and the ability to recognize the shapes that the human brain possesses.

3D captcha looks like this:

The user must find three images offered to him in the panel.

The implementation of this system is not complicated after you have 3D images that you can take yourself or use in a license-free stock of images: The recording program knows the number of proposed images and the number of panel images, this is enough to check that they are the same, while robots need to compare only images.

Questions

An effective alternative to robots that register on forums and place ads is a script that asks a simple question to the user to answer.
If the list of answers to questions is personal, bots have no chance of passing this dam.
This system was included in phpBB.