Hmm, first of all the bbPress stylesheet shouldn’t be loading on sites that do not have bbPress activated. If it is, then there is a problem some where, because it shouldn’t be.
If you want to customize the bbPress CSS, you’ll want to:
Copy:
wp-content/plugins/bbpress/templates/default/css/bbpress.css
to:
wp-content/themes/[YOUR-THEME]/css/bbpress.css
When bbPress runs, it will load your theme’s bbpress.css instead of it’s own.