Refusal to cancel contract!

I signed a two year contract in August 2012 and paid the contract happily until it's end in August 2014. I did not realize that Virgin Active kept charging my account until January of 2015. In February of 2015 I cancelled my contract with Virgin Active ensuring everything paid up. Since then I have received at least one call a month asking when I will be paying my contract and now I have been handed over to a collection agency for amounts that I do not owe Virgin Active. As stated I canceled my account in February, I do not owe this company anything in the following months. Please investigate this matter as I do not like being harassed for money I do not owe.