From Amazon Seller Central:
New: Immediate disposal option for customer returns
Fulfillment by Amazon (FBA) sellers in North America have a new option for disposal of customer returns, saving removal costs and storage fees.
When you enable Automated Unfulfillable Removals, you now can choose to have unsellable customer returns disposed of immediately, rather than on a weekly, twice-monthly, or monthly schedule.
Immediate disposals will be free of charge through March 19, 2019.
To learn how to enable immediate disposal, visit Remove inventory automatically.