Changing this code to only do email

I found this code at dagondesign that will stop people from being able to use a registered name or email when leaving a comment. I was hoping someone could modify it for me so that it only checks the email. I don’t want people with the same names not being able to comment, I do however not want them to use the email of registered users. Mostly because it will assign that persons gravatar and they can also maliciously abuse the “sign me up for email comment responses” I have setup.

Thanks!