@jcvc Hi, sorry to hear that you have been caught out by one of these payforit scams, first thing is to check through the texts and find STOP message number, check that you have some credit and send that message.
I know you don't remember subscribing but that is not unusual, what everyone you and I know consider subscribing is totally different from what these company take as a subscription.
You can follow up with a call to the companies customer service line and request a refund, telling them that you didn't knowingly subscribe for anything.
One other thing you can do is vote for the idea in labs, to get giffgaff to implement a 2FA procedure for payforit"services" which is what EE have been pressured into doing by their customers.
Also if you search for payforit in the forum you can find some very useful threads posted about the subject