Impossible to cancel your account

I used Openweb and signed up on there website and used their direct debit. I cancelled the service on their website and via email to their cancellation department on the 20th of July 2015. I noticed this morning again a direct debit came off my account and now they claim to never have received my cancellation mail. They now want to to cancel again and give 30 days notice. They say an account is only cancelled once you receive a cancellation reference but I sent everything i needed to. If I didn't get a reference then that is their error, why did they not send it as I emailed twice and phoned and cancelled on their website. I forwarded them the email to themselves showing my details and previous cancellation, my email went through but they apparently never got it, it just disappeared into the galaxy. I do not want to send a new cancellation letter for fear that I will be debited again next month. I would've recommended them to anyone but if you cannot even close an account when you need to then what is the point..