Reliance Communications — charging me on free usage

I told the company to close my account on a telephonic conversation, but when i did they offered me a free monthly 2gb without rental plan from 5/04/2013 to 5/06/2013. i got to know about this in april end or so. i started using the offer but never exceeded 2gb. Bill for april was not generated but for may the generated my bill. when i called they said they have charged me rental(for 2gb plan) . i argued with the same and told them to close my account. within 10 days my account was closed. but i was getting mails repeatedly for my due bill. i called up once again and the guy assured me that yes they have done a mistake n he will look in the billing department. after 15-20 days i got a letter from the company about my due bill n dey mentioned that they will take legal action if i ddnt pay them. i called up one more time shouting at them and the lady over the phone said that yes u have been wrongly charged but as u have closed ur account u have to go to a reliance store, i argued and told to transfer my call to a senior representative. But the guy was like, no sir you havent been given any such scheme, now its almost 20 days i am getting a msg from the comapany about the legal action.
Please help.
Thank you