Do not be disheartend. Stay your course, trust the process-- potential guests believe what is consistant when reading reviews. E.g: Clean; Freindly; Welcoming; Comfortable; Beautiful. They might read, but will see through a shallow guests nit-piks.
If you must respond to a foolish, whiny guests bad reviews, do it breifly and without appolgy:
"It is unfortunate (guest name) did not find the accomodations to be on par, or consistant with the reviews of past guests. We wish (guest name) well in her travels, and hope when she finds that perfect place to stay, she will let us know, so we can reccomend it and perhaps visit there ourselves."