That's not good at all, Just keep ringing them until they get fed up, and when I mean keep ringing them, I mean as soon as you speak to someone and they fob you off, ring them back as soon as the call ends
It irritates them and gets them off there backsides, that's what I did with EDF when they were sure I owed them a grand when the bill was actually £17!
But keep ringing and if you get nowhere go down the complaints procedure and if that doesn't work I would go to Ofcom or trading standards or something like that for miselling.