I'm just gonna throw this out there--are you sure the sales change is because of the bad review? It could just be a coincidence (as a statistician, I can tell you that you don't have enough data
). The better reviews outweigh the poor one; most people ignore a single bad review; and a positive review is the first one they see.
Did you do anything else that could affect your marketing at the same time? Were you previously active on a forum and have been less so lately? Were you getting book blogger attention, and now it's died down? Has a competing book gotten a lot of press lately? Think through all the things that might affect your sales, and consider that that one review might not be at fault.
--Maria