By law, any organisation using a 'short code' text number (i.e. with just 5 digits) must cease sending those messages if you text 'STOP' back to that number. I'm not guaranteeing that it will definitely work but the text will only be at your standard message rate so, for the sake of a few pence, it's got to be worth a try.
Otherwise (or as well) try a complaint to Phonepay Plus (which is the relevant regulatory body):
http://www.phonepaypl...Make-a-complaint.aspx
Chris