This article outlines how to archive/delete/remove an active client Voucher/Gift Card
Note: Removing a voucher will cause it to become inactive, but will not free up the Serial number to be used again. There is no way to free up a used Serial number.
Go to Manager - Gift Cards / Vouchers
Single-click on the checkbox to the left of the voucher you want to archive, then click Archive at the bottom of the screen.