Is it a customer or not? The difference is quiet big, if it is a unhappy customer complaining ... not much you can do, if it is not a customer then we are walking in the defamatory field and you can "contact" the site that hosts the comments (if it's a blog, then the blog owner, if it is a forum then the forum owner etc) to bring down the defamatory comments unless there is a real proof placed by the commentator, i am more then sure the site owner will be happy to collaborate. Simple..clean..and works.
If u dont find any way or if u dont happy with replies here..i suggest contacting Cyber crime..they can help u well..yeah