BuddyPress snippet to remove "affiliate" users from the list of active users.

gistfile1.php

<?php

//Code is completely untested, may contain errors.

function affiliate_user_stealth_mode() {
    if ( ! is_user_logged_in() )
		return;

	$user = wp_get_current_user();
	if ( get_user_meta( $user->ID, 'affiliate', true ) ) {
		remove_action( 'wp_head', 'bp_core_record_activity' );
		delete_user_meta( $user->ID, 'last_activity' );
	}

}
add_action( 'init', 'affiliate_user_stealth_mode' );