Not fixing their error

I rented a car in March, they blocked 2 amounts on my account making my account go into negative. When I returned the car the person assured me that they will unblock the second amount that was done in error and it would be returned to me within 2 days. That didnt happen. I called the call centre and have been dealing with them for 2 weeks with no joy. I still dont have my money and no one is calling me back. Ive left my number asking for a manager to contact me and nothing. Every time I call I get assured my money will be unblocked but nothing gets done even though their merchant confirmed the funds are still blocked. Its been 3 weeks.