It's not possible to cancel goodybags once they have activated. You can only cancel queued goodybags.
If the goodybag you bought has queued then you can cancel it by raising a case with the agents.
To cancel a queued Goodybag you have to ask an agent. You have the option to refund to the card used to purchase it or have it added to your airtime balance.
You will need to do this at least a day before queued Goodybag is due to activate as you need to allow for the agent response times. Link to ask agent http://giff.ly/agentscancelqueued