You can't find it because it doesn't exist. You cannot leave a negative for a buyer. You also can't leave a negative comment in a positive format. If you do, it can be removed and you would get a violation.
You can choose to leave no feedback at all.
As pburn states, you need to open and close Unpaid Item Disputes. Closing them gives the buyer a strike, and you get your fees back. Just two strikes will keep them from buying from most sellers who block those with 2 or more strikes in a year.
