At this point, it’s not possible to remove a key permanently for security reasons. However, there are two ways to achieve a similar effect:
Navigate to the Security page and select Hide Blocked Keys (remember to save changes). In order to ‘remove the key’, you simply Block the key on the product page (i.e. by clicking on No in the Block column).
Delete the entire product. It’s a good idea to have two products, one used in production and one used in testing. You are only charged for products that are intended to be used in production.