First, many websites will not remove content without a court order to do so, and a court will not often order the removal of content until a suit has been filed, evidence presented, and/or a lawsuit won.
Even when the terms of service of a website prohibit the posting of defamatory content, truth and opinion are protected forms of speech. A website is generally not in a position to tell truth from falsehood or opinion from fact, and most websites will not want to delete potentially protected comments or criticisms until a court has determined the content is defamatory.
Content posted on public review websites can be problematic, as most review sites are looked at by visitors to the site as "opinion" sites and not necessarily posted facts. It takes a review of the review and its context, and the extent of reputational injury (if any) that has been caused by the written words, to determine if there is a suit worth pursuing.
A "failure to provide services" could be viewed as defamatory, depending on what services are involved and the duty or responsibility of the targeted person to provide these services. And, again, it would depend on how exactly the review is worded.
What you really need to do is have an attorney in your area look over the review and review site, to better determine if there is any legal action available to pursue and, if so, whether a threat to sue based on this available legal action could be enough to have the content voluntarily deleted by the reviewer.
I have listed some sites below which provide general information that many find useful when researching defamation law (most are media-based, but give good overviews of the law). For advice specific to your own situation, however, it still would take an attorney's personal review of the facts particular to your case.
Reporter's Committee for Freedom of the Press, rcfp.org
Knight Citizen News Network, kcnn.org/legal_risk (click on the Rules and the "read more")
Electronic Frontier Foundation, eff.org
Citizen Media Law Project, citmedialaw.org
Good luck.