With a sub 700 credit score, you might not want to simply stop payments.
However, if you are having a service issue, i.e. the product is not being delivered, do as GizmoTX suggests. Force them to show they can make their product work. If they cannot, you are off the hook. If they can....well, then you need to find the original agreement. At worst, it should, like phone plans, have a cancellation fee.
Last ditch idea, claim you are moving. Most services like this have a clause that lets you out if you move.