Hide Comments with Leaky Paywall
You can use Leaky Paywall to hide or restrict comments to only logged in subscriber. The display of comments is controlled by your theme, so you will probably need to edit your theme files.
Here is an example of how you can adjust the single.php file of the twentytwelve theme to hide/show comments using the leaky_paywall_user_has_access() function.
Here is a nice example of a CTA you could display to non-subscribers in place of the comment box.