End of contract

I signed a 1-year contract with virgin active in horizon Roodepoort Nov 2018 until Nov 2019. I called them and explain that since my contract end I will not renew it. The lady on the phone explained that I should tell them 30 days before the contract ended. Which didn't make sense to me. I told her I will only gym for Dec 2019 thereafter and done. Now I get calls from virgin active claiming that I owe money and they calling non-stop and this is quite annoying.