1. config register member need to confirmation link with email ,
2. config Email template to repelace default.
Users will not be able to become new users
if you want to block registering you need to edit you theme so it will not appear in it , no extensions etc .
in your theme in : /app/design/frontend/[theme]/Magento_Theme/layout/default.xml
you need this :
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="register-link" remove="true"/>
</body>
</page>
and registration is gone .