You can use any of this code in robots.txt

User-agent: *
Disallow: /*comment

this will neglect every url that contains 'comment'

or You can use this also

User-agent: *
Disallow: /comment/reply

or

User-agent: *
Disallow: /comment

To neglect every url after '/comment'

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

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.