YOU are the only one that can stop these texts.
The only share of the proceeds giffgaff get is the 10p cost to send the stop message.
If you havent already done so
It is important that you do send the STOP text to 83463 or that you get them to unsubscribe you.
You can even call them on 0333 003 0599 using goodybag minutes.
If you don't do so the messages will still be sent, using up any credit you have, and even if you don't have credit the charges will still be applied when you next add air time credit.
Simply blocking the number will not stop the texts or charges as they are charged at network level, blocking simply stops them appearing on your phone but you will still be charged
You can enter the 5 digit number here http://psauthority.org.uk/about-us/number-checker to find alternative contact methods
See this thread for more advice on how you became subscribed