Try it. (I think you can still do it I am not sure)
you should always take screenshot when you are taking orders which means client requirements and show him if he wants more later.This way you are safe.Also contact fiverr support with the documents.And if possible try to cancel order mitually. In this way your account won’t be affected.
I think I remember dismissing a request to cancel an order.
Try it and then do as @miiila suggested. I have also put off a customer that booked my proofreading gig to do homework for them. I simply thanked them for the order. Then I contacted customer service and asked them to cancel the order. They canceled the order, and it did not affect my stats.