Money gone from account, cant close due to money in account

I over paid my credit card some time back going a year or two. When I asked to close the account they said they couldn't because there was still funds owed to me, if I wanted it closed I had to either use the money or provide banking details. I had already destroyed the card and had requested the money back. I received a reply that they wanted proof of payment which I cannot provide as I cant remember when where or how I paid. The statement has shown the credit up until the statement I received in November 2018. In January I received a statement where the money has gone. I called the call centre but due to high volumes I never spoke to anyone. In February I received the next statement and the same happened, cannot speak to anyone due to high call volumes. Today 14/03/2019 I have received a statement and called, I spoke to a friendly lady but she can only pass on the information and I must wait again.

If you can't give me back my money but can just write off my money isn't that theft?

I'm so disappointed with all this. A fight over R50 is really pathetic. I also want the account closed, I'm tired of not wanting to use you guys due to arrangements not being met on your side.