No, you can't search by author; it's all text based. If you happen to know that they always sign their comments by some unique words, you could perhaps search for that -- but it'd still be very hit or miss (and most people don't have a unique sign-off like that).

Also, comments are only indexed if they are on a paid account. Not if they were made BY a paid account -- but if they are ON a paid account. This is mostly designed for large communities that want to be able to search through comments, as a lot of the content is in the comments.