AMAZON PRIME FREE SUBSCRIPTION TRAP

We live in South Africa. On November 26 my wife logged onto Amazon and purchased gifts for my granddaughter who lives in the UK. Unbeknown to my wife she automatically received a free one month trial of Amazon prime. On 27 December our credit card was debited. I have been struggling with Amazon to get this resolved. Everyone please be extra vigilant when purchasing on Amazon as you may unwittingly be signing up for a subscription you do not require.