Negative feedback doesn't stop buyers from buying or winning auctions.

That's why ebay introduced the strike system which has been in place for years.

Open non payer cases each time and close them 4 days later if they don't pay.

Buyer gets a strike, you get your fee returned and can re-list. They can't leave feedback after that.

Also set your blocks to block serial non payers. Buyer with 2 or more strikes won't be able to bid or buy.

Hover over account tab and select site prefs then buyer requirements.

To the right select show n edit, set blocks and save.