Basically the easiest way to do this is to edit the the topic.php template and check to see if the user is logged in before the loop that shows all the posts.
if they are not logged in, hop over the loop and output a message they must log in.
This could also be done somewhat by a plugin.
This is just a quick and dirty example that might not work as is:
<?php
/*
Plugin Name: Only logged in can read topics
*/
add_action('bb_head','logged_in_topics');
function logged_in_topics() {
if (!is_topic()) {return;}
if (!bb_is_user_logged_in()) {
echo "Sorry, you must register or login to read topics.";
exit;
}
}