I think by now it should be painfully obvious to everyone that the number of people who care about this issue is ridiculously small so the idea or hope that GR will do anything to alleviate those concerns is without foundation. The question remains how does a reviewer prevent or discourage an author from making a comment on a review. It would seem the tools available would be blocking, flagging, or ignoring. One non-standard suggestion might be to simply add a statement at the beginning of each review, "No author comments, please," along with a similar statement in your profile.
Clearly positions on this issue have hardened beyond alteration so my particular bias will be of little interest. Nevertheless, it being a snowy and cold day, here' my two cents. I think that when one discriminates against a category of people, in this case preventing authors from commenting on reviews of their books, they are being denied their ability to speak, but most importantly the reviewer denies him/herself access to the individual who knows the most about the subject, i.e. the book's author.
We have many interesting and creative reviews on GR. When those reviews are created and published on GR, I submit that the reviewer becomes an author, yet I suspect no one would ever propose denying the author of a review the right to comment on his/her own review, yet that is precisely what is proposed when the suggestion is made to prevent an author from commenting on a review of his/her book.
If you are uncomfortable with having an author join the conversation about his/her book on your review, you can certainly warn him/her off with a statement at the beginning of the review. I would hate to see GR or any site formalize something like that. I would also hate to see the day come when authors, who create the very basis for our existence as reviewers, become 2nd-class members without the same privileges as others. There is great value in diversity.