Harassed for payment that we don't owe

We signed up our son, who was 4 at the time, for their kids gym. We were told that we could pay monthly in case he didn't like it. After the second time at the kids gym our son refused to continue going. A year and a half later we still have debt collection agents phoning us weekly, asking for my son (who is now 6), and insisting that he owes virgin for a years subscription to the kids club... At some point Virgin agreed that there was nothing owing and we thought that was then end of it. But no, it seems that their debt collectors did not get the memo and the phone calls and sms's keep coming, week after week, after week.... It really is harassment now.