I had a buyer from **** (strict buyers) for logo project. Everything was done successfully and we both were happy. I was little away from work and during that period same client sent me a message as he did not get any response from me. He messed up my rating and changed it into negative feedback. Well, after lot of requests and assurances he managed to change it but I dont know what happened to him, he again changed positive to negative I think fiverr should give one chance to buyer for edit/remove feedback. This way buyers will not be able to blackmail innocent sellers.
You can contact customer support and request them to look into your case.
Had a similar experience with an ****** buyer for scraping project.