Trustpilot works a little like mob insurance... their strong SEO performance and open platform allows anyone to leave reviews. So competitors use it for "negative PR". Trustpilot have no intention of fixing this as it allows them to sell their platform to potiental clients. The subtext is clear "we will not review malicious and slanderous reviews until you start paying for the service" or they encourage you drown-out the bogus reviews by encouraging geniune customers to start using their platform, rather than a platform you use that only asks genuine customers to provide reviews.