If a currency has been used in an order or an invoice then it cannot be deleted. The Delete button will be unavailable.
If a currency has not been used in an order or invoice but it has a history of previous exchange rates then you can DEACTIVATE it. You can then REACTIVATE it later if you want to use it again.
If a currency has not been used in an order or invoice and there's no history of previous exchange rates then you can DELETE it.
To delete or deactivate a currency:
When you first open the currency page, by default the Currency code dropdown only lists the active currencies. You can click Include inactive to list inactive as well as active currencies.
To reactivate a currency: