Clicking on this flag will trigger a new browser window hosted directly by Google. The page presents a short form to report a policy violation so that the review is removed.

Simply select the primary reason, supply your email, click the box to confirm you are actually a person (and not a bot), and submit. You will receive an acknowledgement from Google that the request as been submitted.

If the content of the single review contains a direct and flagrant violation of Google's rules, this first step might be all that's necessary to have the review removed.