Just put a "badge" next to companies known to sue employees who post reviews on Glassdoor.
No reviews are even needed from that point on to signify a shitty company; for all anyone knows all negative reviews may have been taken down through legal action, but the badge stays, signifying that there was at least one review so damaging that they had to get lawyers involved (or that the company climate is of that toxic kind that sues its own disgruntled employees more generally).
In the end suing will end up acting as the ultimate bad review for that company on the site.