Threat of negative online review

Here's a question I have been pondering now that online reviews (Yelp, AVVO, Google, etc.) are ubiquitous:

Scenario:

Client's case ended satisfactorily 6 months ago or longer;
Client posted a positive review after the case to an online review site;
Client emails asking a "quick question";
In that email client states "Considering this is merely a continuation of the facts of my case, I am assuming you will continue to give me advice thereon";
I write back stating that if they are seeking information about modifying the settlement then we must begin again anew, with a new retainer;
They state that they don't want to retain me to begin a new case, they merely want advice on how they could do it;
I tell them I cannot give them advice on a new matter without being retained;
I get a "Thanks for nothing reply";
Now I am worried that, considering they posted a positive review after their case and that they are clearly web savvy, they can basically make up an infinite number of email addresses to disparage me on these sites with negative reviews, all because I did not want to work for free...


Is there any way to counter this without working for free?