nothing is recorded in the activity stream for blog post categories – there is a reference to the blog_id -> item_id and post_id -> secondary_item; maybe some sort of custom code to look-up and determine the category and not display it (via has_activities filter)?
or; I don’t know if this is possible – try blocking the db update via the action save_post with a higher priority function to remove this action (since save_post will pass along the $post_id and $post – then check for category and remove).
add_action( 'save_post', 'bp_blogs_record_post', 10, 2 );
