Close the Registration of a Wiki
Steps
Stop visitors from creating an account themselves, so that the wiki only holds the accounts an administrator opened. It is done by allowing the "Register" right to the administrators, which, as Rights Resolution explains, is what refuses it to everyone else.
- Administer your wiki.
- Select "Users & Rights", and then "Rights".
- On the "Groups" tab, allow "Register" for XWikiAdminGroup, the way Set the Rights of a Wiki describes.

- Open the registration page in a private browser window, and check that the visitor is asked to log in instead of being offered the form.

FAQ
How does anyone get an account now?
An administrator opens it for them, as Create a User Account describes.
Is allowing "Register" to the administrators not already implied by "Admin"?
"Admin" does grant "Register", but a right that comes with another right is never denied to the others, so leaving the tick out leaves everyone able to register. The tick is what closes the registration, not the right it grants.
Can the members create the accounts instead of the administrators?
Yes: allow "Register" to XWikiAllGroup in step 3 instead of to XWikiAdminGroup, and visitors are still refused while every registered user keeps the form. Wiki Access Configurations lists it beside the other two.
Can I close the registration of one page only?
No. "Register" is set on a whole wiki and on nothing smaller, as Right Types lists.