Using phpMyAdmin, browse your database table:
coupon_gv_queue

and find the GV for the customer and you need to mark the:
release_flag

as X

What this does in keep a record of the GV but not release it to the customer ... now it will not show but if you need to reference it you can see it in the database and in your Admin under the GVs sent ... it will show as not redeemed ...

If you do not want this record of this transaction at all, you can delete it from the:
coupon_gv_queue