You can't delete costing codes from the expense system due to auditing purposes but you can hide them so users are not able to select or see these codes.
Go to Settings/Costing Import
Go to Costing Template on the right side of the screen to download it
Include all existing Costing records click on "YES"
You would then open the downloaded Excel file
In the column C Hide field, next to the Costing code you want it to be hidden put Yes in the box of the Hide column.
Do this for all costing records you want to hide and save the Excel file.
Then go back to Settings/Costing Import and import the created Excel file and click on Yes.
At the end, you should receive the message: Costing Import Successful.