Elizabeth Wein has had a similar problem, but from the other end: an Amazon reviewer posted a vitriolic critique of her novel A Coalition of Lions (though rather confusingly he attached it to the Amazon page for one of her other books), accusing her inter alia of "running interference" for the Eritreans in the recent war with Ethiopia (this in a historical fantasy novel set about 500AD). Wein did the sensible thing and vented about this on her livejournal rather than engage the reviewer directly. Karin Calvo-Goller could learn a thing or two from her.
I've had a few cases of this myself. Some of you will remember my exchange with John Scalzi about his Old Man's War (starting here), as a result of which I modified my opinion of Scalzi's personal politics (having fallen into a similar trap to Elizabeth Wein's Ethiopian critic), though I stand by my view of the book. Three years ago, I reviewed an academic book whose author was upset by my closing paragraphs and wrote to me to say so (and had also written to the editors of the journal where I published it). I responded, rather than defend my criticisms, by pointing out that my review began with three paragraphs of unadulterated praise for the book; and the author replied saying that she would not worry about this any more. I have to say that the epsiode lowered my opinion of her, if not of the book.
Earlier this month I discovered that one of my book reviews on goodreads.com had attracted comments from a Balkan nationalist who took issue with my politics as expressed in the review - indeed, he posted over 7000 words in response to my 300 word piece (which was written in 1998 and has been on-line in several places for over a decade without attracting this sort of attention). Of this massive screed, only one sentence actually referred to the book; the rest was dedicated to proving that Whyte Is Wrong And Evil. (Amusingly, I have also been vigorously criticised from the opposite side on this particular issue in the past.) After some reflection, I decided that I would delete the comments and block the commenter; frank debate is one thing, but this was effectively vandalism of my review.
I draw a couple of general conclusions from these incidents. One is that, as a reviewer, it is always better to play the ball rather than the man (a phrase I picked up from the rules of debate on the excellent Slugger O'Toole group blog about Northern Irish politics). The Irish activist who objects to Scalzi, or the Ethiopian activist who objects to Wein, risks making the mistake of thinking that a fiction author, who is after all writing things which are not true, cares as much about the historical truths as we do, and that we can engage them in debate on the same terms as we do our fellow activists. It is never a bad idea for the reader to sit back and work out why their own baggage affects their reception of the book, before posting that entry or sending that message. Sometimes of course the reader has strong justification; while I acquit Scalzi of deliberately mocking the Irish peace process, it is pretty clear by his own account that he did so unthinkingly (and I still believe that his uncritical portrayal of his unreliable narrator is a failure of art).
Secondly and more pragmatically is the question of ownership of online space and control of online debate. Writers and others can reply to me in comments here, but in the end this is my journal, and I decide what comments get to stay and what are deleted; and likewise in any other space which I control. That's simply a statement of fact. People who object to what I say have every right to do so on their own webspace, and sometimes do; and sometimes I am unwise enough to comment back even if it is obviously a waste of my time (and of course they have the right to delete my comments if they feel like it). The editor of the EJIL is in a slightly different position, as the rules of academic reviewing are more formal; note however that he did offer a right of reply to Karin Calvo-Goller, which she did not take up.
(On a related point, I have been trying to get to the bottom of the Italian court ruling against Google, and rather get the feeling that there is more to the story than meets the eye.)