I had a similar experience with Sky broadband, they sent me an invoice for two years of internet at an address I no longer lived at, called debt collectors, send possibly false solicitors letter etc.
I had already told them about their mistake and they just carried on as the auto respond emails couldn't work out how to reply to my complaint, and as I wasn't even a sky customer anymore I wasn't allowed to talk to a real person.
Only way I got around it was by sending an email to Sky CS directly (even though they say you can only talk to the solicitors) explaining that they were in the wrong, the only thing that stopped them was me threatening to take them to court for harassment if they contacted me again.
I have no idea whether I even could do that or not, but was 100% in the right all along and was at my wits end trying to get them to stop, its surprisingly stressful being chased by debt collectors for no reason!