After four days you can cancel the transaction for nonpayment to give the buyer a strike and prevent feedback from being left. If you cancel earlier or for other reasons, the buyer will not get a strike.
Add the user to your Blocked Buyer List to prevent that particular user from bidding on or buying any more of your listings, regardless of what other sellers do.
Be sure to set your Buyer Requirements to preemptively block bids or purchases from users with two or more unpaid item strikes in the last year. There is also a setting to require a payment method in advance for bids or offers, though it is not clear if that is applied to all possible bidders or offerers.
You can avoid unpaid items by listing your item for a fixed price with immediate payment required and by not accepting offers.
