Validating the identity of each reviewer. If they post something slanderous then the business owner should then be able to get a warrant/court order etc to force the site to reveal the reviewers identity. So long as anonymous posting is allowed there is no way to have a truly reliable platform for reviewing anything.

IDing users wouldn't be too hard to implement. There are plenty of businesses that provide instant ID verification services, many of the new online banks and money transfer apps use these services. The two issues would be cost and the fact that the number of people using review sites that do this would be pretty low.