You will be stopped from buying the £25 goodybag if giffgaff feel that you are abusing it and you ignore the text message(s) asking you to change how you use it.
Turn off recurring goodybag and auto topups, and remove all payment methods [link]
Taking your number with you:
Dial 43431 or go to http://giffgaff.com/help/pac-code-request to request a PAC (Porting Authorisation Code)
The PAC will be delivered by SMS within 2 hours and displayed on your My giffgaff [link] page, under your number.
The transfer must be initiated within 30 days of requesting the PAC, otherwise it will expire and you will need to request another. Please be aware that mobile networks are not obliged to provide a PAC once your account with them has been terminated.
The moment you port your number away your giffgaff SIM card will be deactivated and you will permanently lose any remaining credit, goodybag and/or payback on your account.