I had this problem and it was not resolved satisfactorily.  I could not turn off the monthly billing from my iPad.  I had checked a week before my month expired and there was no way to opt out.  So I figure it would expire and then I would re-sign up.  

At the 30 day point I got billed again, so I decided to turn it off from my iPad.  There was no way to opt out on my iPad, only could I opt in.  

I called Apple they said call ATT.  I call ATT and explained the problem to CSR in Columbia, South America and they said they would call me back.  They never did.  So I went to the local ATT store and had them turn in off. I was on day 31.  They did not refund my cost and I have an ATT account on my phone.  Talk about feeling hosed..........