Outstanding balance

I had an account with them while I was a short while Wembley employee then I paid off my account and then it was blocked from me using it, I got offered a permanent position and tried to unblock my account then they gave me a new card which I had never used because I still needed to send proof of employment to get the account working again, I sent them the contract which they said was invalid so I asked for my account to be closed then they said it would only be closed after I paid the outstanding balance which was apparently for the replaced card I never used, I had previously paid off all the outstanding balances I had but now they want me to pay for a card I never used and they say I’ll get charged interest if I don’t pay it.