You could try to go chapter by chapter, or using the milestone feature (if you have it yet), if you get a new customer for a whole book.
Serious buyers may well accept that, as it´s a security measure for both seller and buyer.
If a buyer starts to get funny after just one chapter, not so much is lost, and if they get funny only after the last chapter, well, if they accepted and reviewed and didn´t cancel the ones before, their request for cancellation becomes very implausible.
For the current case, you could try to prove to Fiverr that you delivered what was agreed on, and of course give them screenshots of the insults etc.
That the person´s account was gone after the cancellation, that might have been not the buyer deleting their account but a case of chargeback, in which Fiverr suspends a buyer´s account, there are a few threads with tips on how to handle that, e.g. this and this.