Unauthorised funds taken by Avis

I have hired a car with Avis and have been extending my rental every week, of which I would get a quot on how much it's going to cost. I would deposit the quoted amount to ensure that funds are available. I would then get an sms from my bank saying the funds have been reserved by Avis. This went on for about 3 weeks. At the end of the contract, the debited the full amount on the rental and did not take into consideration of the funds that they have reserved. This left my credit card on a minus with a huge amount. Thank said there is nothing they can do on their side, I have to speak to Avis. The issue hasn't been solved, no one seems to care, they got their money and now they don't care that they left me in an unacceptable state. This has been going on for 2 months and my credit record is messed up due to Avis OR Tambo. Their service is terrible and they don't care about their customers.