Regret it is not possible to get a refund of unused data etc.
It's either use or lose it.
If you have a recurring goodybag or Top Up set remove by going to the following link :-https://giffgaff.com/profile/payment-details
Ensure that you click Save when you have made changes.
If there is no activity on the Sim Card ie no calls made, no texts, no data used the card will automatically deactivated after six months and number recycled.
You can also destroy the Sim Card if you wish
Sorry to see you go, and I wish you luck with your new network.