Planet Fitness at it again

Received SMS on 21 May about extension of contract. Immediately sent an E mail to cancel contract in 30 days. At end of June they claim that I must phone the Call centre and then said they will consider my request , and the thirty days only start at the end of August to be completed at the end of September. To make matters worse I paid for the contact on behalf of my grandson. They also claim I must get his permission to cancel the contact.