Well, unless you bring in a polygraph test or something similar for reviewers you're never going to be able to guarantee that someone isn't going to deliberately and maliciously give a false bad review, or a deliberately fake glowing review to drum up a bit more business. If the approach is "no reviews unless they're all 100% truthful" then you might as well remove the reviews option altogether, 'cos it ain't gonna happen. Sometimes someone is going to have a genuinely bad day, either escort or client (or both), and the client's opinion of the service is going to be bad. That's life. One way around that maybe is to have a "cooling off period" where you can't review someone until say 24hrs after you've met them, so you don't go home and bang out the review while you're still feeling pissed off.
Sometimes as well you'll meet someone who gives genuinely bad service, and so a bad review is justified (met one last week in fact). The whole "I don't want to give a bad review 'cos who am I to negatively impact someone's livelihood" is a typically irish cop-out, unless someone complains then there's no reason for the bad service provider to up their game and the situation will continue. Every escort has the chance to comment/reply on any review she thinks is unfair, and the good 'uns will do so, so you get to see both sides of the story.