The priority is to prevent them taking any more of your money, if you have no airtime credit, but only a goodybag, they cannot take these charges, but they will still build up and the charges will be taken next time you add airtime credit, if you simply block them they'll stiil be sent and the charges applied, but you'll have no information as to where the airtime credit is going or that the charges are building up.
You need to send STOP to the 5 digit number mentioned in the text
You need 10p credit to send the STOP message, but in the message there will be a customer services number which you can call from your goodybag minutes. It is important that you do send the STOP text or that you get them to unsubscribe you. 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 https://community.giffgaff.com/t5/Tips-Guides/Premium-text-charges-do-s-And-don-t-s/m-p/18597940#M145989
for more advice on how you became subscribed
Once you've sent the stop text you'll get a successfully unsubscribed message.
Once you are unsubscribed it's time attempt to recover your money, contact them from the subscribed phone, using the help number found in the text, or through the email or website found from the link above.
Explain you never knowingly subscribed and would like a refund of the charges