Is there any evidence that glassdoor lets employers do this? I think glassdoor actually specifically says that they won't, unless it violates a law somehow. They even specifically state that they "don’t take sides in factual or contractual disputes between employers and reviewers."
Generally employers figure out who left the bad review and threaten legal action to the person who made the review. It isn't Glassdoor that removes the review but the person who wrote it.
Also anecdotal: our employer "highly encouraged" us to write glowing reviews of the company as a marketing effort to make the company look better to potential hires.
https://help.glassdoor.com/article/I-m-an-employer-What-can-...