VOX struggling to cancel account for 5 months

We've had some exchange accounts and a few pop accounts with VOX for more than 5 years. In this time spam emails were sent from our directors email address whereas they keep telling us to change the password. After getting sick of the process we decided to move our service back to our domain provider. We've cancelled our services with VOX in March 2018. Till this day, after numerous phone calls, emails and meetings that were not kept the account is still not cancelled. They are billing us every month still, threatening us with collection agencies after we've cancelled their debit order... Looks like the cancellation department is a third party. They couldn't find our contract, so much so that after one of their agents waited for a month for another to send it they finally reverted to us to get a copy (from 5 years ago) because they couldn't find it. I'm so glad we only had a couple of emails with them because imagine if we had more services what a mess it would've been. Till this date we've been receiving invoices and statements for amounts outstanding from April. We will not pay them as our cancellation was legit and on time.