If you want all links in your posts to be nofollow, you have to use post_text instead of post_author_link
Here you go
<?php
/*
Plugin Name: My own tweak for my forums
Plugin URI: https://bbpress.org/
Description: Fine tuning codes for my forum
Author: Your name
Author URI: http://www.myforum.com/
Version: 0.1
*/
add_filter(‘post_text’, ‘bb_rel_nofollow’);
?>
I guess that’s what most people want to do