How to noindex comment/reply in Drupal 7
You can use any of this code in robots.txt:

User-agent: *
Disallow: /*comment


This will neglect every URL that contains comment.

You can also use the following, to neglect every URL containing /comment. 

User-agent: *
Disallow: /comment/reply




User-agent: *
Disallow: /comment


After you do this, check whether it is working or not by using the Google Robots.txt checker.