Bad admin

I used to be with buzybuy which was good service. I was with them for a few years. Then Vox took over. First they didn't take over the debit order so i had to pay eft. Not a problem just wasnt communicated. Then they send a statement saying if I terminate service I have to pay them for the router, the one Buzybuy installed 3years prior, I queried this and was told it would not be the case. I gave notice for November to be my final month, and low and behold suddenly they managed to run a debit order - without me completing the authorization form. Yet they still send statements monthly adding interest for overdue account stating i am adversely effecting my credit status by not paying. I have contacted them. Sent proof of payment. All for naught as the statements keep coming and the interest keeps growing.