You have two choices. Which you use depends on whether you are prepared to change to a different mobile number from the one you were given when you activated the SIM card, and also whether you have requested to transfer your own number to giffgaff.
Option 1 - Not bothered about the number and not requested transfer of your own number
Raise a support ticket from the account that you activated the SIM on and ask for the account to be cancelled and your money refunded.
Get a new SIM card (£1 or less from a local shop) and activate it on the account you used to purchase the phone.
You end up with a different number and one account.
Option 2 - You want to keep the same number or you have requested transfer of your own number
Raise a support ticket from the account you bought the phone on and ask for the pre-paid goodybag to be cancelled, giving the reason that you activated the SIM on a different account in error. Give the member name and mobile number of the account with the active SIM.
You end up with the same number but two accounts - one with the phone on, the other with your number and goodybags etc.
Replies from agents (allow 24 hours)