We all know how StackOverflow works. You can always ask or answer a question, but other privileges are based on your reputation. Reputation is only gained by creating good questions and answers. It takes work to get a good reputation on StackOverflow.
I actually don't know what Wikimedia has in place, but it could implement a similar reputation based approach as StackOverflow. Of course the algorithm and mechanism would be different since Wikipedia is not a Q&A site.
To make things even harder, they could implement a reputation killer. For example: if user A improved the score of user B, then user B gets nuked because they are one of these PR firms, then user A should suffer a major reputation dive.
While I'm at it... Instead of Wikipedia begging for money once a year, they could implement a wikipedia.com site which has some light advertising. By default all users go to wikipedia.org, but for those who want can manually redirect to wikipedia.com. I would be glad to support such a system.