Once the Discount Codes are generated, you can only delete/modify generated discount codes at Shopify > Discounts.
Select the Discount Code campaign that you had generated with the Bulk Discounts and Sales Manager app and click on View all Codes.
Select the codes you want to delete.
Click on the Actions dropdown button and select Delete codes.
If you wish to delete the whole campaign that had been generated, just select all codes and proceed to Delete all the codes.
Special Note: As Shopify will not notify apps whenever a discount code campaign is removed, the discount code will still be shown at the Discount Code Generator.