You don't even need to pay Glassdoor. At $PREVIOUS_EMPLOYER I managed to get a negative review by an employee who we'd let go removed just by asking them.
Don't be too horrified yet because it gets worse: I stand by asking them to remove it. The review was extremely negative and posted by somebody who, in the less than 3 months they worked for us, had been nothing but trouble: lazy, arrogant, obnoxious, demanding, complaining, entitled. Nothing was ever good enough for this person, and they absolutely slagged us off in the review as a result.
No company is perfect but the criticism was entirely unfair, so I asked Glassdoor to remove it, explaining why in reasonable detail. They could have refused and there would have been nothing I could do, but within a few days it was gone. Funny thing was that I wasn't even in love with $PREVIOUS_EMPLOYER myself at the time[1], but the review was so out of order that it simply pissed me off.
Your situation sounds quite different, with objective grounds for grievance, so I can certainly understand the frustration.
[1] This is something of an understatement: I was by this point hugely disillusioned, but the review still offended my sense of fair play.