I have just found out that my daughter subscribed to this so called service a couple of weeks ago - thankfully she is on a PAYG service so she lost a relatively small amount of money.
I tried sending the JOKE STOP message to 85050 and received a return message telling me that she was not subscribed to this service, they also charged 50p for the privilege. After asking my daughter, I found that she had activated the service with the word FUN.
I sent the message FUN STOP to 85050 and received a return message informing me that the service has been stopped. Hopefully this will help other users.
PS the web site that appears to be promoting this, is sms4luck.co.uk