NuDebt debiting my bank account 4 years after account confirmed closed.

I cancelled an account administered by Nudebt in Dec 2016. I have a zero balance statement as well as a letter from Nudebt confirming that the account with them is closed. Nudebt has started debiting me again. I have to reverse each debit through my bank. I have tried calling Nudebt this Saturday morning (in their business hours) without the phone being answered.