If you're using Unpaid Item Assistant, it automatically blocks Feedback from buyers who haven't paid. As soon as it opens a case, the buyer won't be able to leave Feedback for your transaction until the case is closed and you've received payment.
If a buyer doesn't pay, an unpaid item is recorded on their account and any Feedback they left for your transaction before the case was opened is removed. If they haven't left Feedback yet, they won't be able to once the unpaid item is recorded on their account.
However, buyers can leave Feedback in these situations:
Before Unpaid Item Assistant opens the case
If they pay after the case is opened
If they successfully appeal after the case is closed (any previous Feedback left will be reinstated)
If an unpaid item wasn't recorded on their account after the case closed