Ok, listen. You have over 600 reviews. Stop worrying about negative reviews. Those buyers are taking advantage of you and you are simply allowing them to do it. How should you handle it?
If you do the work then you should deliver the order. If they threaten a bad review if you do not cancel then you report them to customer service as they are not allowed do that.
It does not matter if they found someone else, they ordered from you, you did the work now you should get paid. Simple. You might get a few negative reviews but so what? In fact, when you get a couple of negative reviews, the scamming buyers will not target you because they know you will not cancel.