There appears to be an increasing amount of people being caught with the text premium scam. Thanks to people like @sloz there are some really helpful posts to read that should help the victims. Thankfully I haven't fallen victim to it myself but from the advice given you need to text stop which will cost 10p. As I understand it if you have nothing in your balance they can't take the money but as soon as you top up again they will take it. Not everyone keeps a small amount in their balance all the time so if you have nothing then obviously you can't text stop. With £10 being the smallest top up to deposit in your balance , surely this feeds the scammers further. Are giffgaff doing anything to help these people that have no balance with the 10p needed for the text? I realise that none of this is giffgaff's fault but are they helping people to top up with a smaller amount to be used for the stop text.?
You have identified a problem which has already arisen a couple of times. It would be good if, as a goodwill gesture, GiffGaff could credit people in this situation with 10p which they could use to send a STOP text. Alternatively they could provide a facility to do a smaller top-up. Unfortunately they are totally disinterested in these scams and rarely do anything to help.
I always advise consumers to phone the company instead of sending a STOP text. The companes are required to operate a helpline number which must be an 01,02 or 03 number which can be called using Goodybag minutes. There is often an automated unsubscribe option. You also need to speak to the company, as even if no money has been taken (because you have no airtime credit), there may be pending charges which need to be cancelled.