This is EXACTLY the same problem I'm facing! I have completed 1,200 orders and have 60 order cancellations, that's 5% cancellation ratio which doesn't sound too good. Aside from a handful of incidents, wherein there was a disagreement in the middle of an order and either I or the buyer asked to cancel the order, the vast majority of these cancellations are from buyers who placed an order without contacting me in advance. They just see a $5 gig price and try to "steal" my work for that price, regardless of the scope of the work they want me to accomplish. That happens regularly when my gig's description first line is:
PLEASE DO NOT PLACE AN ORDER BEFORE CONTACTING ME IN ADVANCE.
(Highlighted in yellow, underscored, all capital letters in bold font - How the hell can a buyer miss it!?)
The correct way of handling such unwanted orders is to contact Fiverr Support and ask them to cancel these orders. If you do it on your own from the Resolution Center, it will negatively impact your gig. I got this confirmed more than once by Support that the healthiest way to cancel an order is to contact them.
There should be the seller's option to accept or deny an order before buyer's order actually becomes active. It's especially in those cases that they click on the Purchase button without bothering to read the description of what they are actually buying. It would save so much headache and hassle for sellers later on.