Ridiculous contract

I signed a contract for 3years from august in 2015 to august 2018 for the Sebokeng gym in Germiston. I went there twice and did not like the setup as it was initially promised there will be a steam room and boxing lessons but that never happed. I tried to cancel but was told to settle it. I let it run through and the last payment was done in October 2018. And after that I stopped the debit orders because my contract was over. Now I have lawyers phoning me that I owe R1600 because I did not owner my contract as I was supposed to write a letter after the contract to say I dont want it anymore yet my 3year contract was finished. Why must I cancel a contract that is already run its course. These people are criminals enriching themselves giving me an excuse that the contract says I must write a letter of cancellation. Right now I am not employed, I have 3 kids to feed. They sent me a message that I am gonna be listed as a default on the credit bureaus. Please help on how I can fix this.