Unathorised debit order on bank account

Nimble has made a unauthorised debit order on my account after I have settled my finchoice account in full.

I have been calling sending emails to the refunds department and customer service but am not getting any assistance.

It's been a month now still I have not received my money even after sending the proof of payment and bank confirmation letter with ID copy as requested.

I have had enough, I want my money back.