Unfair Contract cancelation policy.

Due to ill health I I have had to cancel my contract with Zonefitness by a request from my doctor and specialist physician . I have submitted a letter from the doctor stating that due to having a degenerative Spinal disorder and having chronic deep vain thrombosis and IBS it would be detrimental to my health to continue with the gym. I have sent an email and the letter to their head office and they want me to either pay the monthly installments or a penalty of R1250 to cancel the contract. I have refused to pay either as I am cancelling my contract due to ill health and the recommendation of my doctor. I am not prepared to pay for services which will not be rendered due to ill health. I am now being harassed by Zonefitness with legal action if I refuse to pay.