How Do You Remove Comment Subscription?

A person is complaining about receiving email every time a new comment is posted on the post he subscribed to, and there is not unsubscribe link at the bottom of the email, how do i Remove him from the email subscription list of comments for posts? I already tried going to Tools>Subscription and his email is not under that, where would I go?

Thanks
PS: this is a private hosted blog but i assume it would still be the same, can you advise?