What are the cancellation terms of the contract? In most cases, if you do not cancel within the cooling-off period you're at their mercy.
Closing the account won't make the debt go away, they could then send collection agencies after you.
Your best bet is to provide evidence that they are not providing the service you signed up for, and get out of the contract that way. But generally, you can't just decide you don't want it any more, send back the equipment and not pay what you contracted to.