Spammers and other people you do not care to link to often attach their URL to the website field of your comments. If you are concerned with sending out "link juice"... Don't have much faith in the nofollow tag? Why not get rid of the option all together? You can keep the field on your form, but when the comment is approved, it will just show the text of the author's name (no link).

Here is how its done:

wp-includes folder > comment-template.php

Search for:

$return = "<a href='$url' rel='external nofollow' class='url'>$author</a>";
Replace with:

$return = $author;