How and how to protect the site from "alien"?

Question

CAPTCHA

There is a task to create a public highly specialized wiki site for everyone who knows the subject area.
The site will be free, registration is the most common.

Part of the information - will be available only to authors working on content.

How can you immediately weed out all those who came, as it were, to "help" in creating content, but perhaps not quite in the subject (regarding the subject area), and pursuing other goals?

The very principle of weeding out such "authors" - what would you recommend?

PS: exclusive referral registration by invitations of acting authors is not an option.

Answer 1

Solution of the problem on the topic of the site. In some mathematical forums, it was proposed to analytically find the derivative or indefinite integral.

Answer 2

Thanks everyone for the replies!
In general, the juice is like this ( Bavashi suggested the idea - Thanks to him for this!):
1. You need to mirror the desired wiki branch of the main "tree" of the wiki onto a degenerate "tree" for each participant and a demo article (sample 2-3 design examples different types of pages) - leave it available to everyone.
2. Tests - let the very first 3-5 participants create BEFORE the main wiki.
PS: I'll mark this answer as a solution if it gets a difference of more than 5 likes inclusive compared to the top answer already marked as a solution .