You could always use a consultant like the one at badreviewhelp.com
on cruise critic, i've seen the owner come in behind and post follow up comments.
What the heck is yelp?
There are always people that you just cant please,.. you knowmthe circumstances. I wouldnt worry too much as your satisfied cutomers would outway this guy. And if you are worried he will spread it around, others will already know what an A hole he is and probably ignore it. Plus you dont want any more customers like him to walk through your door. My ex boyfriend worked in advertising, they used to say any publicity was good publicity,.. so at least it may make people wonder what your business is all about! LOL