Contract

Last year my wife took out membership at virgin active to use their water aerobic facilities. She suffers from rheumetoid arthritis and the water aerobics brought some relief. However her health deteriorated to such an extent that she was no longer able to attend these sessions. We approached Virgin Active with a request to cancel the contract. This was promptly refused by Virgin Active in George ( western cape ). What are our rights in this regard? Are consumers obliged to pay for a service they no longer require?