deleting comments in bulk
AFAIK, VBO should be able to do the job, but I haven't tried it myself to delete comments.

Another way is to run some code (in a module or in Devel module's 'execute php' block) that gets a list of all queries from the db, builds an array of the comment IDs and passes that array to the comment_delete_multiple function. Note that this can take a while to complete (depending on server performance and the number of comments), so you may have work your way around that, for instance by using set_time_limit (http://php.net/manual/en/function.set-time-limit.php) or Drupal's batch API.

[update: see Chris Cohen's answer for example code based on this approach.]