Plusnet Grrrrrr

We both took out some deals with Plusnet approx 3 months back, on receiving the SIM cards we found the signal was very poor where we lived,it wouldn't work indoors and was almost as bad outside. After discussing this with Pluset on the same day we received the SIM cards they agreed to cancel them both and refunded our deposits. We are still getting bills from them, mine keeps either telling me I am in credit and no payments will be taken, my partner's keeps telling her that she has not paid and they keep adding it on every month. They also say log on to check our accounts,we have tried and as I thought it doesn't recognise either of us. I have tried several times to call,but keep being kept on hold in long queues,which I am paying for. On the website they give the option of writing your complaint and sending it by post,where they PROMISE to respond by email or letter within 10 days,that's been about 28 days now with no response. We both know we have no accounts,there is no direct debit set up,if we had accounts we would have been receiving threatning letters by now demanding payment and stopping the SIM card . I see they have been in trouble recently with customers accounts. Do I just ignore these emails,or do I go to the ombudsman,it may sound trivial,but it's very annoying,and points to a very shambolic accounts department. We have been broadband users with them for about 5 years now with no issues at all, and are now also thinking of moving that as well,as this issue and there awful customer service has annoyed us so much. What would you do, and NO I WONT PHONE THEM.