There are a couple of ways to cancel a GV ...

1 login with your own test account and claim the GV
This method prevents fully deleting the GV for record keeping and you could make your test account just for deleted GVs so that you can track how many are manually claimed.

2 login to you phpMyAdmin and find it in the table:
coupons
and edit it and mark the field:
coupon_active
as: N
The record will still be there but will be marked as in active so it cannot be claimed ...

3 The least desirable method is to delete the record in the database table:
coupons
This destroys all ability to track the GV in any manner ...