there's no option available to flag author comments. Authors are allowed to say whatever they want in response to your reviews because, in the end, it's their own work. You can choose not to watch their work, or to vote lower on his submission if you are annoyed by how he responds to reviews.
Flagging reviews is primarily to protect authors from abuse. And, since review moderators don't have the tools to delete only an author's response, it would be counter productive to delete legitimate reviews just because the author left a mean comment on it. And, it could open up room for authors to manipulate the average review score of his flash by leaving nasty comments to low scoring reviews.