How can I prevent visitors on the wiki to register themselves?
This was previously done through a config option named “openregister” and was replaced by the general disableactions option in the recent stable release. Just disable the register action in the config manager like this:
If your configuration is read-only to the user running your webserver, you may edit your configuration directly:
$conf['disableactions'] = 'register';
to disable open user registration.
You might also consider allowing open registration, but making new users powerless until approved.