Hide bbPress topics / replies / forums from public or non-logged in users.

functions.php

function pj_hla_logged_in_topics($have_posts){
    if (!is_user_logged_in()){
        $have_posts = null;
  	    echo 'Login to see replies';
    }
    return $have_posts;
}

add_filter('bbp_has_topics', 'pj_hla_logged_in_topics');
add_filter('bbp_has_forums', 'pj_hla_logged_in_topics');
add_filter('bbp_has_replies', 'pj_hla_logged_in_topics');