discount code used that isn't in available

[VDEV]

A customer used the discount code "lifestyle270" for a discount on an order.

Issue is, we never had such a code available, active, or otherwise.  It took off more than 15% of the order and there is no way for this code to be stopped from being used in the future as we have no access to it.

Why is this something that can happen and how can be stopped????