As you've been with giffgaff so long then you know prices have to rise, at least you're not in a contract where you have to stay whilst paying the higher charges.
Go to https://giffgaff.com/profile/payment-details and
Cancel / turn off any auto top up
Cancel / turn off any recurring goodybag
Remove your credit / debit card detailsScroll down and ensure you save the changes
Log out and back in to ensure the changes are saved.
If you take your number to a new network then your giffgaff sim is deactivated, but deactivates after six months of non use anyway.
If you need to keep your number with another network you'll need a PAC code.
There are two ways to get the PAC code
1. Dial 43431
2. Click here https://giffgaff.com/help/pac-code-request
Your PAC code will arrive as a SMS within 2 WORKING HOURS (Monday-Friday, between the hours of 8AM and 4PM),it will also be visible on your dashboard
Your pac code is valid for 30 days