Late joiner penalty fees

I joined Discovery Health on 2010/05/01 and in my application form I mentioned that I was part of a medical aid before I joined, then Discovery send me a letter to confirm that a penalty will be imposed and that I must sign the letter before I can join the medial scheme. I signed the letter and the penalty was imposed. I queried the penalty fee after 5 years and was informed that the penalty was imposed because I was not on a medical aid before joining, I advised them that I was on a medial scheme and the penalty was removed, but when I requested the backdated penalty fees to be refunded because I was charged incorrectly I was informed that they will not pay back the money. They did not have a problem taking my money, but when it came to pay it back.