Yeah when it comes to a bad review you either have to be completely mute, or just say "thanks for your time, and I'm sorry it didn't work for you." As part of my "Writing Advice" Series I wrote up a blog post about dealing with reviews. The reality is that most books, especially those that have any kind of commercia success aren't "bad." They just aren't what the reviewer was looking for, or as in the case here, might have made a decision that you don't agree with. The other thing you start to realize (once you get reviews in sufficient number) is that what one person loves, another hates and vice versa. I wrote a pretty entertaining blog post on this a while back.