I had a similar thing happen to me about 2 years ago with a customer from SM. He bought some cheap cameras on Ebay and had us install them.  6 months later one of them died, I came out at no charge to him, diagnosed the problem. Told him to send the camera back.  

He gets the new camera, calls me to install it, and I told him that I'd do it at the regular hourly rate.  He didn't like that and left a Bullsh@t rating on SM.  The minute I saw it I called SM.  They basically told me I was screwed. They told me  I could respond to his review and that was that.  

They said I could try and contact him to resolve the issue, I told them that there's no way I was going to call him and get extorted for labor, because there is NO issue with my installation, it was crap product that I didn't provide.   

SM said they would investigate it, and in the meantime, they'd remove the review temporarily.  30 days later it popped up again, I called again.  They said they couldn't get in contact with the customer, he wouldn't return messages so they re-instated his review.  Wha?  

In a nutshell, I told them if they didn't remove it immediately, I was going to cancel my account.  At that point, I had a rep turn my account off until until they got it resolved.  Oddly, that took less than 24 hrs.  

I completely agree with you though, they really do have you by the shorthairs with those reviews. The rep told me, one of the reasons they removed it was because of the vast majority of our ratings were 5 stars.