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 web-server, 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.