Disallowing anonymous posting

I'm building with WP 3.2.1 running MultiSite and BP 1.2.10. I have tested the Comments Plus plugin extensively; so if I'm wrong about what I'm posting here below my apologies in advance for being such a dork.

From what I gathered, this is how the plugin works: If you disallow comments on a post (either per post or by default), the Comments Plus form will not show up at all. If you allow comments and the user is logged into your site, the form includes the regular WP logged-in comment form, plus the three tabs for the three external services, in case the user would rather use those.

If you allow comments on a post and the user is *not* logged into your site, you'll get the tabs for the three services, and the standard WP *anonymous* comment form. BUT: If you do not allow anonymous posting to your site, the Comments Plus form will not show up at all.

What I would like is to disallow anonymous posting sitewide - but use Comments Plus to allow it through its three external services; so that the non-logged-in user would see the three tabs for the services, but NOT the WP anonymous comment form - maybe instead a message that says, "You can also comment by logging into this site. Register here"... etc.

Would it be easy to change the script to allow this? Or have I gotten this completely wrong? Oh Incsub Gods, do tell... My thanks in advance --