Under Ofcom rules, all companies offering text-based services must cease sending you texts if you simply send the word 'stop' to the relevant number. That text should not be charged at a premium rate. (i.e. only your normal text charge will apply). The regulating body, PhonePayPlus, can take measures to disconnect any text service which fails to act after receiving a 'stop' text but, in the first instance (if your 'stop' text is ignored) you should follow the advice here:
http://www.ofcom.org....ptions/?itemid=285859
(Click 'More options' for details of how to contact PhonePayPlus
Regrettably, the Telephone Preference Service has no control over SMS messages. However, if you've not already registered your mobile number with them, you should still do so to bar advertising voice calls:
http://www.mpsonline.org.uk/tps/
Chris