Just don want me to be on prepaid

I had a contract with this service provider that was supposed to send at the end of July. For two months now they have me on a month to month contract after I had advised that i'm not interested in it. I just want to be on prepaid and not on any of their contracts. Why is it so difficult to cancel my contract. I was willing to take a data contract but after the service I got i'm not interested in any of their contracts.