Most wiki farms are open. This is for flexibility, and ease of setup. But with a little work they don't have to be.
The main thing you need to create a closed farm is a list of wiki names, and the email addresses of the people that should own them.
The first step is to make sure only the right traffic can reach them. This should be done on the server using a reverse proxy. One easy way to set this up is to use Nginx like you would for a virtual host setup. You can then direct only the domain names you want at your server.
Would a list of approved farm host names be a good feature to add? <-- I think so. So I proposed a way to add it. -- forage.ward.fed.wiki.org
Mozilla discontinued "Persona" authentication service
~~Then you have to create the persona.identity file for each wiki, so that they can't be claimed by anyone else. […]~~
See Mozilla Persona