MBD AND VIRGIN ACTIVE !

I am absolutely upset about MBD and VIRGIN ACTIVE! when I was 15 years old my grandfather had a membership at virgin active and I used to visit the gym under his membership for free since I was UNDER 21. My grandfather was then in arrears with his payments and stopped going to the gym, then a year later MBD started calling me and harrassing me over the phone saying I am liable to pay and my virgin active account has been handed over to MBD ATTORNEYS. Now at the time I was only 15 and definitely did not agree to a membership and it is all on their system. I spoke to my grandfather about this and he also said that he freed that account months ago and the proof is on their side but they are still holding me liable and contacting me, no one wants to help me at all and my name should not go bad because of their mistakes!