As we know every link on your website will cost you sooner or later rankings in Google, at least if you have a lot of them. Much worser are links to bad neighborhoods, if you link to them you risk penalties by Google too.
A forums has normally lots of links, just imagine a member has posted a link to some site he is working on at the moment of posting and this site will become a bad neighborhood after a couple of month.
This is why people should change their links on forums. In Wordpress is this a default behavior, all comment links are "rel=nofollow". The bbress software has this function too, as a bbpress forum owner you need to enable this option.
Place this function into some plugin file and enable the plugin (if not already done) and all links are changed.
function post_with_links_nofollow( $post_id = 0 ) {
$post = get_post_text($post_id);
return bb_rel_nofollow($post);
}
add_filter('post_text', 'post_with_links_nofollow');