It's frustrating to get a bad review, but it happens to most
authors sooner or later.
My advice for authors is to try not to take it personally
and to remember that books (especially fiction) are subjective - some people
will love your book and others won't. And many book buyers realize that most
books will have some negative reviews, even if most of them are positive.
If a review contains factual errors (for instance stating
that a nonfiction book was missing important information) you can click the
"comment" button on the review and leave a note. But be very careful
not to sound defensive - just state the facts. (e.g. Perhaps you missed chapter
6 where I discussed that topic in detail.)
If you think a review was really unjustified, you can also
click the "no" button next to "Was this review helpful to
you?"
If a review violates Amazon's terms you can ask Amazon to remove it. Amazon will not remove a review simply because
it's unfavorable or you think it's unfair, but they sometimes remove reviews
that are reported for violating their terms.
Also focus on getting more good reviews to offset the bad ones.
And finally, take an objective look at negative reviews and
see if there are any legitimate comments that you might use to improve your
writing.