Just remove all bank details and any recurring goodybag.
For goodybag guidance please see below.
Your SIM card and number will deactivated after 6 month of none use. The account will be still there to order another SIM with new number if you wish to come back. Or contact an agent if you want your account removing for good.
Sorry hope this help.
@leannestuart05 To add to what the other giffgaffers have advised about removing auto top ups/recurring goodybags and payment details, any airtime credit/payback points on account will be lost when the sim deactivates, so use any airtime balance (if you have any) beforehand, also the username leannestuart05
Will remain should you wish to return to giffgaff and activate a new sim in the account.