A few of the websites I manage have suffered vast accumulation of spam comments (just under 10,000 in one case). Mollom and ReCAPTCHA are shockingly easy to mis-configure. Deleting 10,000 comments 50 at a time via the Drupal 7 comment moderation form is not how I would like to spend my time; here is the easiest solution I have found:
In Drush, run the following SQL query to delete all unpublished comments:
drush sql-query "DELETE FROM comment WHERE status = 0"