If the order has been completed to the scope purchased, then decline any cancellation requests put forth by the buyer. I won’t say it’s an empty threat, but cancellations are not as easy to make as a simple click of a button. The buyer would have to go through Customer Support to cancel without your permission, and as long as you’ve met all the criteria and complied will all Fiverr standards, you should be fine.
The suggested reply from @zerlina84 is pretty good, but I would recommend making it a statement, not a question.
IF you’re willing to do one more free revision:
You have used the 3 free revisions provided in your purchase. As a courtesy I will provide one more at no charge. All future revisions after this will cost $X.
IF you are not willing:
Thank you for your continued interest in my service. As you have used the 3 free revisions provided in your purchase, all future revisions will cost $X. I have sent the revisions charge for you to accept or decline as you will.
Kudos for wanting to stand your ground. Stand Your Ground: Setting Boundaries as a Seller
Has the buyer given off any other warning flags? Anything else that has made you question their authenticity? If so, you could reach out to CS yourself, emphasize you do NOT want to cancel, and state your concern that the buyer is trying to get more work than agreed.