add_filter( 'widget_tag_cloud_args', 'jmw_exclude_tag_from_tag_cloud');
function jmw_exclude_tag_from_tag_cloud( $args ) {
  $args[ 'exclude' ] = '45'; // ID of the tag. If multiple tags use comma delimited sting '2,5,36'
  return $args;
}

Exclude tag from the WordPress tag cloud widget