With Membership: How to Show Topics but Hide Threads in a Forum?

Hello,

I am using Membership Lite for a Mingle Forum where users have to register and pay for membership. We want non-members to be able to see topics (and the rest of the WordPress site) but not the threads/actual messages (and we don't want them to post, of course). Right now, Mingle Forum either gives a 404 for the forum page or redirects to another page without showing any of the forum threads. How can I do this with my current setup, upgrade or other WorddPress plug-ins?

Cheers and thanks.