Resetting the Easystore Coupon Codes (once used)

Hi Folks, i got several Coupons in the easystore and deleted a few of them. Today i wanted to set an "old coupon" back as new coupon - but it wasn´t possible, because the code was already in use in a former coupon. (Couponname and Code are supposed to be exactely the same as the old coupon) --- How can erease / get rid of the old codes? Where can i find them ?
Thx in Andvance