Wordpress - How do I delete all comments from a specific old blog post?

Alternative for people reading this with a fear for SQL............... (or finding this via Google after Januari 2011):

Wait for this action until 3.1 comes out, then go to a post, check all comments and bulk "move to trash" :) (it should come out any day now) (http://wordpress.org/about/roadmap/)

(or download 3.1 RC3 from http://wordpress.org/download/release-archive/)

Example:

http://edward.de.leau.net/files/bulkremove.png

WordPress doesn't support bulk management of comments on a post by post basis. Although it does have a comment management section directly at the post's edit page. As you said, you would have to click on "Trash" on every single comment:

Alternately you could take a back up of the wp_comments and wp_commentmeta table and run the following queries:

1. Find post ID. Lets say the relevant id is X.

   SELECT ID from wp_posts WHERE post_type='post' AND post_title='Hello world!' INTO @x; 
   

2. Add relevant comment metadata just to preserve integrity:

   INSERT INTO wp_commentmeta (comment_id, meta_key, meta_value) SELECT comment_ID, "wp_trash_meta_time", UNIX_TIMESTAMP() FROM wp_comments WHERE comment_post_ID=@x;
   
   INSERT INTO wp_commentmeta (comment_id, meta_key, meta_value) SELECT comment_ID, "wp_trash_meta_status",comment_approved FROM wp_comments WHERE comment_post_ID=@x;
   

3. Trash all comments:

   UPDATE wp_comments SET comment_approved='trash' WHERE comment_post_ID=@x;
   

Hi @Jeff Atwood:

I'm assuming you have MySQL query access. This will give you all comments for your blog post whose URL slug is 'your-blog-post' (the slug is the last segment in your post's URL if you are using pretty permalinks, i.e. for http://example.com/2011/01/foo-bar-baz/ your slug would be 'foo-bar-baz'):

SELECT * from wp_comments WHERE comment_post_ID IN (
  SELECT ID FROM wp_posts WHERE post_name='your-post-slug'
)

And this will give you all the comment metadata:

SELECT * from wp_commentmeta WHERE comment_id IN (
  SELECT comment_ID from wp_comments WHERE comment_post_ID IN (
    SELECT ID FROM wp_posts WHERE post_name='your-post-slug'
  )
) 

So... run these two commands (but be sure to replace the post_name value to be equal to yours):

DELETE from wp_commentmeta WHERE comment_id IN (
  SELECT comment_ID from wp_comments WHERE comment_post_ID IN (
    SELECT ID FROM wp_posts WHERE post_name='your-post-slug'
  )
);

DELETE from wp_comments WHERE comment_post_ID IN (
  SELECT ID FROM wp_posts WHERE post_name='your-post-slug'
);

P.S. This will of course fully delete them but unless you do want to keep them in the trash this is easier than moving them to the trash.