You can use any of this code in robots.txt:

<!-- language: lang-none -->

    User-agent: *
    Disallow: /*comment

This will neglect every URL that contains _comment_.

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

<!-- language: lang-none -->

    User-agent: *
    Disallow: /comment/reply

<!-- break -->

<!-- language: lang-none -->

    User-agent: *
    Disallow: /comment


After you do this, check whether it is working or not by using the [Google Robots.txt checker][1].



  [1]: http://www.robotstxt.org/checker.html