If you go to the main page of the bill, you should be able to see the $9.99 data charge being billed to the account.
If you go through that persons billing the last page will show what company is billing the premium SMS to you. You can opt-out of the messaging by replying Stop or Cancel to that same code number. When another message comes through you can also reply with Stop or Cancel.
If you'd like, you can even block any future premium SMS from being sent to the phone via http://bit.ly/xB4iTc .
I had the same issue. My spouse receive random text message from Bid and Win, which she did not enroll and she ignore it.
Next month we receive 9.99 charge for premium messaging. When I called verizon customer service, the lady said I should be responsible for it and even she receive similar messages and always reply with STOP. So, without any solution, i was forced to pay it and feel cheated by verizon, since they are charging customer for a service they did not provide.
I reply the initial text message with STOP and receive a stop confirmation.