This functionality is available for PRO version.

The Registration module controls the URL for the registration process and the activation URL on a MultiSite.

New register-me.php

As default, the register-me.php is being used for the registration process, This is the place where users can register a site or / and a ursername. This functionality provides a way to change the default URL to something else.
The registration status can be controlled through the network super admin interface:

Block register-me.php URL

Block the default register-me.php file. If New register-me.php is being used, is save to block the default, the registration process will continue to work.

New wp-activate.php

This is the URL through which a user can activate a registered blog. Using this feature the URL can be mapped to something else.

Help – Block wp-activate.php

Block access to wp-activate.php file. Through this file, new users confirm that the activation key that is received in the email after signs up for a new blog, matches the key for that user.
If New wp-activate.php is being used, is save to block the default, the registration process will continue to work.