I keep track of a lot of sites, and at one point I needed to disable the CAPTCHA on one of them and forgot to re-enable it. Needless to say the spam bots had a field day. I don't check the site very often as I don't have much time to add new content, but when I was there last I realized that there were over 1000 comments waiting for approval. Way too many to scan through 20 at a time to see if there might be some actual people responding. I wanted to review as much as I could, so I took the following approach. I used phpMyAdmin to selectively remove comments.

A lot of spam recently just has the title 'people', so this was a sure bet:

DELETE FROM `comments` WHERE `subject` = "people";

There were a lot of comments all from the same IP address, so this was easy too:

DELETE FROM `comments` WHERE `hostname` = '195.225.177.6';

You can search within the comments for specific words or phrases. Most of them are pretty common. I hit a couple like this one:

DELETE FROM `comments` WHERE `comment` LIKE '%master%';

Finally though there were still over 800 in the table, so I gave up and just deleted all of the unapproved ones:

DELETE FROM `comments` WHERE `status` = 1;

Then I made sure to install the ReCaptcha module to slow them down.