The preview domain is created for customers to use before their DNS is updated - it's only created if you have a staging domain set under settings->platform.
It won't be indexed by search engines unless it's linked from somewhere else.
Currently you can only disable it with the API - we are considering adding a feature to the UI so that customers can remove it.
In the meantime, the easiest solution is a canonical redirect in the .htaccess. Something like:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^yourprimarydomain\.com$
RewriteRule (.*) https://yourprimarydomain.com/$1 [R=301,L]
