you need to contact the 'old people' and get them to cancel their phone line contract whoever it is with
you cannot cancel the line like you they may not be a BT customer and no one here can help you you need your provider to sort it for you
They will soon cancel when they receive their next bill.
That assumes BT knows where to send the bill to. There's the risk that the OP will be fighting off debt collectors in a few months time as BT try to chase up their bills.