Simple plugin for WordPress Multi-site and BuddyPress to restrict categories, by slug, from the Activity Stream. It's a Multi-site plugin, meant to be installed in wp-content/mu-plugins.

buddypress-exclude-category-slugs-from-activity-stream.php

<?php
/*
 Simple, Multi-site plugin for WordPress and BuddyPress to restrict categories, by slug, from the Activity Stream
 Please note, his is a Multi-site plugin meant to be installed in wp-content/mu-plugins/ folder in a Multi-site WordPress install

 code derivative of snippet by imath at:
 http://buddypress.org/support/topic/possible-to-exclude-post-categories-in-activity-stream-and-make-a-user-profile-private-like-with-pri/
*/

function exclude_category_slugs_from_activity_stream( $post_id, $post ){

  // add in any categories to exclue from activity stream, separated by commas, no spaces
	$category_slugs_to_exclude = "testing,autoblog";

	// create the array that contains the category ids to exclude
	$ids_to_exclude = array();
	// create new array by splitting category slugs by comma
	$category_slug_array = split( ",", $category_slugs_to_exclude );
	// iterate over category_slug_array
	foreach ( $category_slug_array as $category ) {
		// get the category id based on the slug
		$idObj = get_category_by_slug( $category );
		$id = $idObj->term_id;
		// push the category id onto the exclude array
		array_push ( $ids_to_exclude, $id );
	}

	// check post status to make sure it's published, exit if not
	if ( 'publish' != $post->post_status )
		return false;

	// get the post's categories
	$categories = get_the_category( $post_id );
	$in = false;

	// check if the post has any categories, do nothing if not
	if( count($categories) > 0 ) {
		// iterate over categories
		foreach ( $categories as $category ) {
			// check if any excluded category exists within post's categories
			if( in_array( $category->cat_ID, $ids_to_exclude) )
				$in = true;
		}
	}

	/* if the post has any excluded category, remove BuddyPress hook that records an activity */
	if( $in )
	remove_action( 'save_post', 'bp_blogs_record_post', 10, 2 );

}

// add our function to save_post action hook
add_action( 'save_post', 'exclude_category_slugs_from_activity_stream', 9, 2 );
?>