The company promoting this 'service' is Sure to Win ltd http://www.suretowin.com/Contact-Us/Sure-To-Win.html, they have terms and conditions and privacy policy stated, but they are responsible for the premium texts. Sponge UK are just the 'technical platform'.
If seeking to stop. Just text STOP. It is an automated service and anything additional will mean that it will not be picked up, if it is picked up anyway!
I can't find a telephone number. Sure to Win Ltd were bound over by the Office of Fair Trading in 2006 for misleading practice with premium phone numbers, they have form. Company personal cited in the OFT press release http://www.oft.gov.uk/news-and-updates/press/2006/105-06 Sure to Win Limited, its officers Charles Bloom and Richard Scott and controller Malcolm Hellewell in rela