bbp_forum_enforce_hidden()

function bbp_forum_enforce_hidden() {

	// Bail if not viewing a single item or if user has caps
	if ( !is_singular() || bbp_is_user_keymaster() || current_user_can( 'read_hidden_forums' ) )
		return;

	global $wp_query;

	// Define local variable
	$forum_id = 0;

	// Check post type
	switch ( $wp_query->get( 'post_type' ) ) {

		// Forum
		case bbp_get_forum_post_type() :
			$forum_id = bbp_get_forum_id( $wp_query->post->ID );
			break;

		// Topic
		case bbp_get_topic_post_type() :
			$forum_id = bbp_get_topic_forum_id( $wp_query->post->ID );
			break;

		// Reply
		case bbp_get_reply_post_type() :
			$forum_id = bbp_get_reply_forum_id( $wp_query->post->ID );
			break;

	}

	// If forum is explicitly hidden and user not capable, set 404
	if ( !empty( $forum_id ) && bbp_is_forum_hidden( $forum_id ) && !current_user_can( 'read_hidden_forums' ) )
		bbp_set_404();
}