How to delete apprved workflow

Dear All,
I have an approved workflow and ended with success.
My customer uses this workflows item_key while creating employees.

But, they say thet they have approved it by mistake and that is why they dont want to use this item_key.
So in any case I see that I have to delete or purge this workflow in order to prevent useles of this item_key

hope I am clearly enough.

thanks in advance.

Best regards
Kıvanc.