Use a filter to edit the get_the_term_list function
<?php
/* Add nofollow tag to all category and tag links on posts*/
add_filter( "term_links-post_tag", 'add_tag_class');
function add_tag_class($links) {
return str_replace('" rel="tag">', '" rel="tag nofollow">', $links);
}
/* End nofollow tag edit */
?>