BuddyPress.org

Like I said, as long as your custom BP CSS file exists in your theme – /wp-content/themes/YOUR-THEME/buddypress/css/buddypress.css, the one bundled with BuddyPress will not load – /wp-content/plugins/buddypress/bp-templates/bp-legacy/css/buddypress.min.css.

If you don’t even want to include a custom BP CSS file in your theme, you could try using wp_dequeue_style( 'bp-legacy-css' ).