Make a personal attempt to contact the source and listen to their claim. Once you’ve diffused their emotions and reassured them you are sincere, it may be possible to resolve the issue with the source and acquire a retraction.

The first steps are to remove your emotion, do the research, compose yourself, make the call, be compassionate and consider their point-of-view, reach-out and be who you really are (the sincere legitimate authentic business person you are). Avoid going on the defense or over-reacting. Take methodical pre-planned baby-steps.

Of course this advice is subject to re-evaluation depending on the circumstances; fraud is a serious claim with serious consequences and may require legal advice as well.

My gut feeling is that no matter what the circumstances are, I remain in control, cool, calm and precise in my approach with anyone who may be mis-guided or mis-informed. Generally, this approach often diffuses the immediate circumstances at hand and allows for a conversation, which in-turn propagates mutual respect and understanding.

You’ve got to remove yourself emotionally and listen to your accuser to satisfy their need first. (just my opinion)

You would do well to implement online reputation management tactics that would displace any negative search results whether they are relevant to you specifically or the other site.