According to the card user agreement :
How to Close Your Card.
Simply spend down the amount on your Card, cut it up and
throw it away. You may close your Card at any time at www.turboprepaidcard.com or by
calling the number on the back of your Card.
If you have any questions or need assistance with your Turbo Prepaid Visa Card, please visit TurboPrepaidCard.com or call us toll-free at (888) 285-4169.
Just like all debit cards ... once they are zeroed out and go dormant for a couple of months the account will automatically closed with a zero balance ... I would not worry about it if you stopped using it.