Gift Vouchers via API

I have searched the API docs and asked BC support with no solution as of yet so they suggested posting my question here. Thanks in advance for reading.

I need to some how cancel / delete a gift voucher if that voucher is used via an external application. I want the customer to be able to use this voucher in a retail environment or online but not both.

I have successfully added an order via the API using a gift voucher as the payment type but there is not a field for the gift voucher code being used that I can find so the same gift voucher code remains active.

Any ideas would be greatly appreciated.