Your giffgaff account will be closed automatically if you transfer your number to another provider with a PAC code. Any credit in your account will be lost, so it would be worth using it first.
If you want to close your account and are not transferring out the number, simply stop using the sim. It will become inactive after 6 months.
Check that auto top-up and recurring goodybags are switched off on your payment details tab:
Remove your card details from the same tab to prevent any unexpected payments.
As giffgaff is Pay as You Go, there is no contract to cancel,
Just cancel your direct debit with your bank and use the links above to cancel any recurring goodybags stop using your giffgaff SIM, you will lose any airtime credit unless you use it all.
If you want to take your number with you dial 43431 for your PAC code.
Sorry to hear you are leaving
It's really easy to leave giffgaff mate, all you need to do is cancel your recurring goodybag and remove your bank details from you're profile.
Before I provide you the link, can I ask why it is you're leaving us? Is there anything giffgaff can do to entice you to stay? If not we're sorry to see you leave and here's that link I promised: