If you have the Auth0 Welcome Email template enabled (status toggle turned on), then the welcome email will be sent automatically when a user is registered.
Unfortunately, you can’t prevent the welcome email for some users and not others, even within a hook or a rule.
What you can do is disable the Welcome Email template and then set up your own email endpoint to send Welcome Emails. You could add a key such as welcome_email_sent in the users’ app_metadata to keep track of whether the user should be sent a welcome email like in the example here: Customize Email Handling