Student Account being charged

Im actually so furious of this Bank because while I was a student I opened up a Student Account which is valid from the age of 16-24. However I stopped to use the account 2017 without going to the bank to close it cause I havent reached 25 years yet. So Last month I got Messages from the Laywers claiming that I owe R4127 which I dont know for what was it about. I went to bank to ask but No one assisted me. They just told me I was charged R69 with Interest but dont know for what reasons. Because the Laywer wanted to take further action against me I had to pay R3501 which was calculated at the percentage of 15% of what i owe the ban*****il Now I still dont know what the Charges where for since It was Student Account which is Interest free. I never got Calls, Messages or Mail from the bank until I got the Smses and Phone calls from the Laywers