In v15, select "Options - Company Setup/Customization - Company Information and Settings" from the main menu. Click on the Defaults tab and look for the entry that says "Credit/Payment Application" and select Apply Credits from the list.
With this default enabled, it will prompt you if you want to use an outstanding credit when you get to the payment window of a transaction. This will allow you to deplete the credit as the customer purchase additional products. When the credit is depleted, it will no longer prompt you.